Merge bitcoin#32884: rest: replace rf_names[0].rf by RESTResponseFormat::UNDEF

6d19815 rest: replace `rf_names[0].rf` by `RESTResponseFormat::UNDEF` for code clarity (Eval EXEC)

Pull request description:

  I'm reviewing the bitcoin's rest.cpp source code.
  In the function: `ParseDataFormat`, `rf_names[0].rf` is actualy `RESTResponseFormat::UNDEF`:
  https://github.com/bitcoin/bitcoin/blob/e3f416dbf7633b2fb19c933e5508bd231cc7e9cf/src/rest.cpp#L48-L57
  so it would be more clarity and code readability to use `return RESTResponseFormat::UNDEF;` to replace `return rf_names[0].rf;`

ACKs for top commit:
  maflcko:
    lgtm ACK 6d19815
  brunoerg:
    code review ACK 6d19815

Tree-SHA512: 420454f1cc09db44c1d76423d8623a0b8865d41d6c34015844ff83d78a9373e3e26f3f62818d1502b33eb063caf904750e858b74ddecd76750577ae82b64b0c1

Author: fanquake

src/rest.cpp

@@ -144,7 +144,7 @@ RESTResponseFormat ParseDataFormat(std::string& param, const std::string& strReq
 
     // No format string is found
     if (pos_format == std::string::npos) {
-        return rf_names[0].rf;
+        return RESTResponseFormat::UNDEF;
     }
 
     // Match format string to available formats
@@ -157,7 +157,7 @@ RESTResponseFormat ParseDataFormat(std::string& param, const std::string& strReq
     }
 
     // If no suffix is found, return RESTResponseFormat::UNDEF and original string without query string
-    return rf_names[0].rf;
+    return RESTResponseFormat::UNDEF;
 }
 
 static std::string AvailableDataFormatsString()