[JENKINS-45519] Fix keepUndefinedParameters option for suppressing warnings (#2939)

* Update ParametersAction.java

Pull request 2687 added a 'false' setting for this flag to prevent warning messages from being logged, but the logic doesn't match the message or the documentation. This updates the check, so that the warning message is correctly suppressed if `hudson.model.ParametersAction.keepUndefinedParameters` is set to false.

* Updated check to use optBoolean

core/src/main/java/hudson/model/ParametersAction.java

@@ -316,8 +316,8 @@ public class ParametersAction implements RunAction2, Iterable<ParameterValue>, Q
             return parameters;
         }
 
-        String shouldKeepFlag = SystemProperties.getString(KEEP_UNDEFINED_PARAMETERS_SYSTEM_PROPERTY_NAME);
-        if ("true".equalsIgnoreCase(shouldKeepFlag)) {
+        Boolean shouldKeepFlag = SystemProperties.optBoolean(KEEP_UNDEFINED_PARAMETERS_SYSTEM_PROPERTY_NAME);
+        if (shouldKeepFlag != null && shouldKeepFlag.booleanValue()) {
             return parameters;
         }
 
@@ -326,7 +326,7 @@ public class ParametersAction implements RunAction2, Iterable<ParameterValue>, Q
         for (ParameterValue v : this.parameters) {
             if (this.parameterDefinitionNames.contains(v.getName()) || isSafeParameter(v.getName())) {
                 filteredParameters.add(v);
-            } else if ("false".equalsIgnoreCase(shouldKeepFlag)) {
+            } else if (shouldKeepFlag == null) {
                 LOGGER.log(Level.WARNING, "Skipped parameter `{0}` as it is undefined on `{1}`. Set `-D{2}=true` to allow "
                         + "undefined parameters to be injected as environment variables or `-D{3}=[comma-separated list]` to whitelist specific parameter names, "
                         + "even though it represents a security breach or `-D{2}=false` to no longer show this message.",