fix upcoming lint avoid_returning_null_for_void (#23190)

dev/bots/run_command.dart

diff --git a/dev/bots/run_command.dart b/dev/bots/run_command.dart
index 6b8f689..c996aae 100644
--- a/dev/bots/run_command.dart
+++ b/dev/bots/run_command.dart
@@ -44,7 +44,7 @@
   final String relativeWorkingDir = path.relative(workingDirectory);
   if (skip) {
     printProgress('SKIPPING', relativeWorkingDir, commandDescription);
-    return null;
+    return;
   }
   printProgress('RUNNING', relativeWorkingDir, commandDescription);