Style: Replaces uses of 0/NULL by nullptr (C++11)

Using clang-tidy's `modernize-use-nullptr`.
https://clang.llvm.org/extra/clang-tidy/checks/modernize-use-nullptr.html

main/tests/test_shader_lang.cpp

@@ -303,7 +303,7 @@ MainLoop *test() {
 	if (cmdlargs.empty()) {
 		//try editor!
 		print_line("usage: godot -test shader_lang <shader>");
-		return NULL;
+		return nullptr;
 	}
 
 	String test = cmdlargs.back()->get();
@@ -311,7 +311,7 @@ MainLoop *test() {
 	FileAccess *fa = FileAccess::open(test, FileAccess::READ);
 
 	if (!fa) {
-		ERR_FAIL_V(NULL);
+		ERR_FAIL_V(nullptr);
 	}
 
 	String code;
@@ -339,13 +339,13 @@ MainLoop *test() {
 
 	if (err) {
 		print_line("Error at line: " + rtos(sl.get_error_line()) + ": " + sl.get_error_text());
-		return NULL;
+		return nullptr;
 	} else {
 		String code2;
 		recreate_code(&code2, sl.get_shader());
 		print_line("code:\n\n" + code2);
 	}
 
-	return NULL;
+	return nullptr;
 }
 } // namespace TestShaderLang