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.

AK/LogStream.h

@@ -49,7 +49,7 @@ public:
 #endif
     {
     }
-    virtual ~LogStream() { }
+    virtual ~LogStream() = default;
 
     virtual void write(const char*, int) const = 0;
 
@@ -94,7 +94,7 @@ protected:
     bool empty() const { return m_size == 0; }
 
 public:
-    BufferedLogStream() { }
+    BufferedLogStream() = default;
 
     virtual ~BufferedLogStream() override
     {
@@ -114,7 +114,7 @@ public:
 
 class DebugLogStream final : public BufferedLogStream {
 public:
-    DebugLogStream() { }
+    DebugLogStream() = default;
     virtual ~DebugLogStream() override;
 
     // DebugLogStream only checks `enabled` and possibly generates output while the destructor runs.
@@ -128,7 +128,7 @@ private:
 #ifdef KERNEL
 class KernelLogStream final : public BufferedLogStream {
 public:
-    KernelLogStream() { }
+    KernelLogStream() = default;
     virtual ~KernelLogStream() override;
 };
 #endif