Fix Template.search() dropping zero-variable matches Template.match() returns an empty dict when a template with no variable placeholders matches a subtree, and None when there is no match. search() tested the result for truthiness, so the empty dict was treated as "no match" and every zero-variable match was silently dropped. translate() therefore never replaced anything for such templates. search() now tests `res is not None`, matching the contract match() documents. Two existing search tests asserted the broken behaviour and are corrected; added a test that a genuinely different tree still yields nothing, and a TemplateTranslator test covering a variable-free translation. Closes #1630
