Fix a crash in parallel mode when the module's filepath is not set

[PCManticore]

Steps

• Add yourself to CONTRIBUTORS if you are a new contributor.
• Add a ChangeLog entry describing what your PR does.
• If it's a new feature or an important bug fix, add a What's New entry in doc/whatsnew/<current release.rst>.
• Write a good description on what the PR does.

Description

This has been like this since the parallel refactoring done in 8babeff

Type of Changes

	Type
✓	🐛 Bug fix

Related Issue

Close #3564

[coveralls]

Coverage increased (+0.0008%) to 90.699% when pulling a28af84 on fix-crash-parallel-mode-missing-file-path into 36250aa on master.

[coveralls]

Coverage increased (+0.008%) to 90.706% when pulling a28af84 on fix-crash-parallel-mode-missing-file-path into 36250aa on master.