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.

Libraries/LibDebug/DebugSession.cpp

@@ -90,12 +90,12 @@ OwnPtr<DebugSession> DebugSession::exec_and_attach(const String& command, String
 
     if (waitpid(pid, nullptr, WSTOPPED) != pid) {
         perror("waitpid");
-        return nullptr;
+        return {};
     }
 
     if (ptrace(PT_ATTACH, pid, 0, 0) < 0) {
         perror("PT_ATTACH");
-        return nullptr;
+        return {};
     }
 
     // We want to continue until the exit from the 'execve' sycsall.
@@ -105,7 +105,7 @@ OwnPtr<DebugSession> DebugSession::exec_and_attach(const String& command, String
 
     if (waitpid(pid, nullptr, WSTOPPED) != pid) {
         perror("wait_pid");
-        return nullptr;
+        return {};
     }
 
     auto debug_session = adopt_own(*new DebugSession(pid, source_root));
@@ -114,7 +114,7 @@ OwnPtr<DebugSession> DebugSession::exec_and_attach(const String& command, String
     int wstatus = debug_session->continue_debuggee_and_wait();
     if (WSTOPSIG(wstatus) != SIGTRAP) {
         dbgln("expected SIGTRAP");
-        return nullptr;
+        return {};
     }
 
     // At this point, libraries should have been loaded