Style: Enforce braces around if blocks and loops

Using clang-tidy's `readability-braces-around-statements`.
https://clang.llvm.org/extra/clang-tidy/checks/readability-braces-around-statements.html

modules/jsonrpc/jsonrpc.cpp

@@ -151,8 +151,9 @@ Variant JSONRPC::process_action(const Variant &p_action, bool p_process_arr_elem
 }
 
 String JSONRPC::process_string(const String &p_input) {
-	if (p_input.empty())
+	if (p_input.empty()) {
 		return String();
+	}
 
 	Variant ret;
 	Variant input;