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.

DevTools/HackStudio/Editor.cpp

@@ -436,7 +436,7 @@ void Editor::set_document(GUI::TextDocument& doc)
         m_language_client = get_language_client<LanguageClients::Shell::ServerConnection>(project().root_path());
         break;
     default:
-        set_syntax_highlighter(nullptr);
+        set_syntax_highlighter({});
     }
 
     if (m_language_client) {

DevTools/HackStudio/Project.cpp

@@ -43,7 +43,7 @@ Project::~Project()
 OwnPtr<Project> Project::open_with_root_path(const String& root_path)
 {
     if (!Core::File::is_directory(root_path))
-        return nullptr;
+        return {};
     return adopt_own(*new Project(root_path));
 }
 

DevTools/IPCCompiler/main.cpp

@@ -337,13 +337,13 @@ public:
                 parameter_generator.append(R"~~~(
         @parameter.type@ @parameter.name@ = @parameter.initial_value@;
         if (!decoder.decode(@parameter.name@))
-            return nullptr;
+            return {};
 )~~~");
 
                 if (parameter.attributes.contains_slow("UTF8")) {
                     parameter_generator.append(R"~~~(
         if (!Utf8View(@parameter.name@).validate())
-            return nullptr;
+            return {};
 )~~~");
                 }
             }
@@ -449,7 +449,7 @@ public:
 )~~~");
 #endif
         endpoint_generator.append(R"~~~(
-            return nullptr;
+            return {};
         }
 
         if (message_endpoint_magic != @endpoint.magic@) {
@@ -460,7 +460,7 @@ public:
 )~~~");
 #endif
         endpoint_generator.append(R"~~~(
-            return nullptr;
+            return {};
         }
 
         i32 message_id = 0;
@@ -473,7 +473,7 @@ public:
 )~~~");
 #endif
         endpoint_generator.append(R"~~~(
-            return nullptr;
+            return {};
         }
 
         OwnPtr<IPC::Message> message;
@@ -507,7 +507,7 @@ public:
 )~~~");
 #endif
         endpoint_generator.append(R"~~~(
-            return nullptr;
+            return {};
         }
 
         if (stream.handle_any_error()) {
@@ -518,7 +518,7 @@ public:
 )~~~");
 #endif
         endpoint_generator.append(R"~~~(
-            return nullptr;
+            return {};
         }
 
         return message;
@@ -543,7 +543,7 @@ public:
                 } else {
                     message_generator.append(R"~~~(
             handle(static_cast<const Messages::@endpoint.name@::@message.name@&>(message));
-            return nullptr;
+            return {};
 )~~~");
                 }
             };
@@ -553,7 +553,7 @@ public:
         }
         endpoint_generator.append(R"~~~(
         default:
-            return nullptr;
+            return {};
         }
     }
 )~~~");

DevTools/Profiler/Profile.cpp

@@ -368,7 +368,7 @@ Profile::LibraryMetadata::LibraryMetadata(JsonArray regions)
 
         auto file_or_error = MappedFile::map(path);
         if (file_or_error.is_error()) {
-            m_libraries.set(name, nullptr);
+            m_libraries.set(name, {});
             continue;
         }
         auto elf = ELF::Image(file_or_error.value()->bytes());