convert: replace .txt extension filter with empty Query filter

[svenrdz]

Resolves #34

The .txt extension filter allowed esgpull convert to be used with a very coarse path, such as **/*.

To keep this feature, convert now compares each generated Query with an "empty" Query that is produced according to default_options set in the config.
Any file that produces that same empty Query is then reject, since we assume this fike was NOT a valid synda selection file.

Changes

• Remove the .txt extension filter
• Reject any file producing a converted Query that matches an "empty" Query