[JENKINS-67175] More gracefully handle empty bundlesToDownload

[jglick]

In a support bundle (ironically enough) I noticed a stack trace

java.util.NoSuchElementException
	at java.base/java.util.HashMap$HashIterator.nextNode(HashMap.java:1513)
	at java.base/java.util.HashMap$KeyIterator.next(HashMap.java:1534)
	at com.cloudbees.jenkins.support.SupportAction.doDownloadBundles(SupportAction.java:227)

which looks like it could come from an attempt to download no bundles. Should be handled more clearly.

[jglick]

Tried to reproduce actually, but without any existing bundles, the button is offered and if you click it you get a 400 browser error, which is not too friendly either

support-core-plugin/src/main/java/com/cloudbees/jenkins/support/SupportAction.java

Lines 192 to 195 in 5a4a228

	if (!json.has("bundles")) {
	rsp.sendError(HttpServletResponse.SC_BAD_REQUEST);
	return;
	}