[JENKINS-42194] Code review reveals valid point, no schedules is not a warning

(cherry picked from commit 5f5f8800)

core/src/main/java/hudson/triggers/SCMTrigger.java

@@ -358,6 +358,24 @@ public class SCMTrigger extends Trigger<Item> {
                 return FormValidation.ok();
             return FormValidation.validateNonNegativeInteger(value);
         }
+
+        /**
+         * Performs syntax check.
+         */
+        public FormValidation doCheckScmpoll_spec(@QueryParameter String value,
+                                                  @QueryParameter boolean ignorePostCommitHooks,
+                                                  @AncestorInPath Item item) {
+            if (StringUtils.isBlank(value)) {
+                if (ignorePostCommitHooks) {
+                    return FormValidation.ok(Messages.SCMTrigger_no_schedules_no_hooks());
+                } else {
+                    return FormValidation.ok(Messages.SCMTrigger_no_schedules_hooks());
+                }
+            } else {
+                return Jenkins.getInstance().getDescriptorByType(TimerTrigger.DescriptorImpl.class)
+                        .doCheckSpec(value, item);
+            }
+        }
     }
 
     @Extension

core/src/main/resources/hudson/triggers/Messages.properties

@@ -23,6 +23,8 @@
 SCMTrigger.DisplayName=Poll SCM
 SCMTrigger.getDisplayName={0} Polling Log
 SCMTrigger.BuildAction.DisplayName=Polling Log
+SCMTrigger.no_schedules_no_hooks=Post commit hooks are being ignored and no schedules so will never run due to SCM changes
+SCMTrigger.no_schedules_hooks=No schedules so will only run due to SCM changes if triggered by a post-commit hook
 SCMTrigger.SCMTriggerCause.ShortDescription=Started by an SCM change
 TimerTrigger.DisplayName=Build periodically
 TimerTrigger.MissingWhitespace=You appear to be missing whitespace between * and *.