ApiHelp: Avoid header for example, when there no examples

When writing a new extension the source:

	public function getExamples() {
		return array(
			//TODO
		);
	}

would produce a empty "Example:" header on the help page. Avoid this, by
changing the condition to check for truthy instead of false and empty
string.

Change-Id: I67ecacbbac66b97ed96412abf79b49aebe0ebdbf