Bugfix: SSViewer check object exists before calling prop or method_exists

[chrispenny]

$on can be null (there is no property type enforcement for item in the __construct()), and when it is the method_exists() call will throw an error (as it expects object or class for the first param).

Another option could be do write it as method_exists($on ?? '', $property ?? ''), but I thought maybe this was a bit more descriptive.

My particular project comes across this issue when we use SSViewerCacheBlockTest::_runtemplate(), but there might be other situations where item is null.