mIF: make Imaging date optional

[roshni-b]

What

mIF S1609 - allow imaging date to be an empty field

Why

MD Anderson doesn't collect this info.

Checklist

Please include and complete the following checklist. You can mark an item as complete with the - [x] prefix:

• Tests - Added unit tests for new code, regression tests for bugs and updated the integration tests if required
• Formatting & Linting - black and flake8 have been used to ensure styling guidelines are met
• Type Annotations - All new code has been type annotated in the function signatures using type hints
• Docstrings - Docstrings have been provided for functions
• Documentation - Documentation in docs/ has be regenerated and README has been updated to explain major changes & new features
• Package version - Manually bumped the Schemas package version in cidc_schemas/__init__.py

[codeclimate]

Code Climate has analyzed commit abae860 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (50% is the threshold).

This pull request will bring the total coverage in the repository to 93.8% (0.0% change).

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