LibCpp: Don't crash on an empty '#' line

Closes #5634

Userland/Libraries/LibCpp/Preprocessor.cpp

@@ -67,6 +67,8 @@ void Preprocessor::handle_preprocessor_line(const StringView& line)
     lexer.consume_specific('#');
     consume_whitespace();
     auto keyword = lexer.consume_until(' ');
+    if (keyword.is_empty() || keyword.is_null() || keyword.is_whitespace())
+        return;
 
     if (keyword == "include") {
         consume_whitespace();