Fix Lua tag_index return values

[wpferguson]

return nil (empty table) for non-existent tag or tag with no tagged images

Fixes #19461

Test script

test_tag_image.zip

I tested on a new install and create a tag before importing any images. The test script tries to access tag at row -1, which doesn't exist. Next it accesses the tag at row 1 and attempts to find any images with that tag (which should be none). Or, you can trust that I tested it :-D

[TurboGit]

Thanks.