AK: Simplify constructors and conversions from nullptr_t

Problem:
- Many constructors are defined as `{}` rather than using the ` =
  default` compiler-provided constructor.
- Some types provide an implicit conversion operator from `nullptr_t`
  instead of requiring the caller to default construct. This violates
  the C++ Core Guidelines suggestion to declare single-argument
  constructors explicit
  (https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#c46-by-default-declare-single-argument-constructors-explicit).

Solution:
- Change default constructors to use the compiler-provided default
  constructor.
- Remove implicit conversion operators from `nullptr_t` and change
  usage to enforce type consistency without conversion.

Kernel/Syscalls/execve.cpp

@@ -27,6 +27,7 @@
 #include <AK/LexicalPath.h>
 #include <AK/ScopeGuard.h>
 #include <AK/TemporaryChange.h>
+#include <AK/WeakPtr.h>
 #include <Kernel/FileSystem/Custody.h>
 #include <Kernel/FileSystem/FileDescription.h>
 #include <Kernel/PerformanceEventBuffer.h>
@@ -269,7 +270,7 @@ KResultOr<Process::LoadResult> Process::load_elf_object(FileDescription& object_
         executable_size,
         VirtualAddress(elf_image.program_header_table_offset()).offset(load_offset).get(),
         elf_image.program_header_count(),
-        master_tls_region ? master_tls_region->make_weak_ptr() : nullptr,
+        AK::try_make_weak_ptr(master_tls_region),
         master_tls_size,
         master_tls_alignment,
         stack_region->make_weak_ptr()

Kernel/Syscalls/read.cpp

@@ -51,7 +51,7 @@ ssize_t Process::sys$read(int fd, Userspace<u8*> buffer, ssize_t size)
     if (description->is_blocking()) {
         if (!description->can_read()) {
             auto unblock_flags = Thread::FileBlocker::BlockFlags::None;
-            if (Thread::current()->block<Thread::ReadBlocker>(nullptr, *description, unblock_flags).was_interrupted())
+            if (Thread::current()->block<Thread::ReadBlocker>({}, *description, unblock_flags).was_interrupted())
                 return -EINTR;
             if (!((u32)unblock_flags & (u32)Thread::FileBlocker::BlockFlags::Read))
                 return -EAGAIN;

Kernel/Syscalls/socket.cpp

@@ -115,7 +115,7 @@ int Process::sys$accept(int accepting_socket_fd, Userspace<sockaddr*> user_addre
     if (!socket.can_accept()) {
         if (accepting_socket_description->is_blocking()) {
             auto unblock_flags = Thread::FileBlocker::BlockFlags::None;
-            if (Thread::current()->block<Thread::AcceptBlocker>(nullptr, *accepting_socket_description, unblock_flags).was_interrupted())
+            if (Thread::current()->block<Thread::AcceptBlocker>({}, *accepting_socket_description, unblock_flags).was_interrupted())
                 return -EINTR;
         } else {
             return -EAGAIN;

Kernel/Syscalls/thread.cpp

@@ -132,7 +132,7 @@ int Process::sys$join_thread(pid_t tid, Userspace<void**> exit_value)
     // NOTE: pthread_join() cannot be interrupted by signals. Only by death.
     for (;;) {
         KResult try_join_result(KSuccess);
-        auto result = current_thread->block<Thread::JoinBlocker>(nullptr, *thread, try_join_result, joinee_exit_value);
+        auto result = current_thread->block<Thread::JoinBlocker>({}, *thread, try_join_result, joinee_exit_value);
         if (result == Thread::BlockResult::NotBlocked) {
             if (try_join_result.is_error())
                 return try_join_result.error();

Kernel/Syscalls/waitid.cpp

@@ -42,7 +42,7 @@ KResultOr<siginfo_t> Process::do_waitid(idtype_t idtype, int id, int options)
     }
 
     KResultOr<siginfo_t> result = KResult(KSuccess);
-    if (Thread::current()->block<Thread::WaitBlocker>(nullptr, options, idtype, id, result).was_interrupted())
+    if (Thread::current()->block<Thread::WaitBlocker>({}, options, idtype, id, result).was_interrupted())
         return KResult(-EINTR);
     ASSERT(!result.is_error() || (options & WNOHANG) || result.error() != KSuccess);
     return result;

Kernel/Syscalls/write.cpp

@@ -97,7 +97,7 @@ ssize_t Process::do_write(FileDescription& description, const UserOrKernelBuffer
                 return total_nwritten;
             }
             auto unblock_flags = Thread::FileBlocker::BlockFlags::None;
-            if (Thread::current()->block<Thread::WriteBlocker>(nullptr, description, unblock_flags).was_interrupted()) {
+            if (Thread::current()->block<Thread::WriteBlocker>({}, description, unblock_flags).was_interrupted()) {
                 if (total_nwritten == 0)
                     return -EINTR;
             }