Merge bitcoin/bitcoin#24632: add (none) in -getinfo Warnings: if no warning returned

0cea7b1 print `(none)` if no warnings in -getinfo (/dev/fd0)

Pull request description:

  Adds `(none)` in warnings when no warnings returned by -getinfo

  Reviewers can test this by making the following change in `/src/warnings.cpp`:

  ```diff
  bilingual_str GetWarnings(bool verbose)
  {
      bilingual_str warnings_concise;
      std::vector<bilingual_str> warnings_verbose;

      LOCK(g_warnings_mutex);

      // Pre-release build warning
      if (!CLIENT_VERSION_IS_RELEASE) {
  -        warnings_concise = _("This is a pre-release test build - use at your own risk - do not use for mining or merchant applications");;
  +        warnings_concise = _("");;
  ```

  Before this pull request:

  ```
  $ bitcoin-cli -getinfo
  Chain: regtest
  Blocks: 0
  Headers: 0
  Verification progress: 100.0000%
  Difficulty: 4.656542373906925e-10

  Network: in 0, out 0, total 0
  Version: 239900
  Time offset (s): 0
  Proxies: n/a
  Min tx relay fee rate (BTC/kvB): 0.00001000

  Warnings:
  ```

  After this pull request:

  ```diff
  $ bitcoin-cli -getinfo
  Chain: regtest
  Blocks: 0
  Headers: 0
  Verification progress: 100.0000%
  Difficulty: 4.656542373906925e-10

  Network: in 0, out 0, total 0
  Version: 239900
  Time offset (s): 0
  Proxies: n/a
  Min tx relay fee rate (BTC/kvB): 0.00001000

  Warnings: (none)
  ```

ACKs for top commit:
  jonatack:
    ACK 0cea7b1
  laanwj:
    Tested ACK 0cea7b1

Tree-SHA512: a12499d11ff84bc954db354f968eb1f5ee4999d8b80581fe0bdf604732b2e2f608cb5c35c4ca8cb5a430f3991954a6207f0758302618662e6b9505044cf2dc95

Author: laanwj

src/bitcoin-cli.cpp

@@ -1055,7 +1055,9 @@ static void ParseGetInfoResult(UniValue& result)
         result_string += "\n";
     }
 
-    result_string += strprintf("%sWarnings:%s %s", YELLOW, RESET, result["warnings"].getValStr());
+    const std::string warnings{result["warnings"].getValStr()};
+    result_string += strprintf("%sWarnings:%s %s", YELLOW, RESET, warnings.empty() ? "(none)" : warnings);
+
     result.setStr(result_string);
 }