False positives for try-with-resources when the resource is definitively null

[gtoison]

When a try-with-resource has a branch where the resource is definitively null false positive NP_LOAD_OF_KNOWN_NULL_VALUE and RCN_REDUNDANT_NULLCHECK_OF_NULL_VALUE are reported, for instance for that code sample:

	try (InputStream input = Issue2836.class.getResourceAsStream(resource)) {
		if (input == null) {
			return null;
		} else {
			return "test";
		}
	}

Fixes #2836