Optiontal paths for IMA file measurements

# Enhancement Description
- One-line enhancement description (can be used as a release note): Ignore the full path of a file when no path is present.
- Keylime Enhancement Proposal: (link to keylime/enhancements file, if none yet, link to PR)
- Primary contact (assignee): @lukehinds
- Enhancement target (which target equals to which milestone):
- Alpha release target (x.y)
- Beta release target (x.y)
- Stable release target (x.y)

_Please to keep this description up to date._

The issue names no files, tests, or entry points, so first clarify the intended behavior for IMA file measurements and identify where that logic lives. Define the expected result for paths that are present versus absent, then add or update coverage showing that behavior.

Written by the indexing model from the issue text.