Check for cached metadata version before comparing it

[jonabc]

Fixes an issue where a new dependency's license metadata was not being cached when version metadata for the dependency was not available.

This bug occurs as a result of #21. Previous to that change, licensed only considered reusing cached license data when the cached data existed. A file existence check was removed to optimize for using cached data for more than version checks.

github/linguist hit an edge case where their license information, which doesn't contain version metadata, began to view non-existent metadata as the same version as new metadata and skipped caching the new content.

This PR adds a check that the cached data contains non-nil version metadata before checking if the version metadata is current. Since we can't verify version information that doesn't exist, nil or empty version metadata will force rechecking and caching license content.

/cc @lildude FYI and review as the reporter of the bug

[lildude]

With a little tweaking of how Linguist calls licensed (to take into account the changes introduced in #33), I can confirm this resolves the problem from a Linguist point of view:

$ script/licensed --module /Users/lildude/github/linguist/vendor/grammars/m3
Caching licenes for linguist:
  grammar dependencies:
caching /Users/lildude/github/linguist/vendor/grammars/m3
    Caching m3 ()
License caching complete!
* linguist grammar dependencies: 1
$ ls vendor/licenses/grammar/m3.txt
vendor/licenses/grammar/m3.txt
$