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/LibRegex/C/Regex.cpp

@@ -69,7 +69,7 @@ int regcomp(regex_t* reg, const char* pattern, int cflags)
 
     // Note that subsequent uses of regcomp() without regfree() _will_ leak memory
     // This could've been prevented if libc provided a reginit() or similar, but it does not.
-    reg->__data = new internal_regex_t { 0, 0, nullptr, 0, ReError::REG_NOERR, {}, 0 };
+    reg->__data = new internal_regex_t { 0, 0, {}, 0, ReError::REG_NOERR, {}, 0 };
 
     auto preg = impl_from(reg);