Code submission error fix

[MP-DG]

Description of changes

Added fix to code submission not saving correctly.

No longer saves files containing null.
No longer saves files with null as filename.

Only saves files with in properties specified endings.

How has this been tested?

Please describe the test strategy you followed. If not tested, please explain why.

• automated unit test
• automated integration test
• manual, exploratory test

In case of manual test, please document the test well including a set of user instructions and prerequisites. Each including an action, it's result, and where appropriate a screenshot.

Checklist before requesting a review

• My code is easy to understand
• My code follows the coding guidelines of this project
• My code fulfills all acceptance criteria
• The test coverage (line and branch) is reasonably high, especially on Service classes and other classes with complex logic
• I have made corresponding changes to the documentation or
  the wiki/adr
• I made no breaking changes in the database schema or if so, I will perform a database migration

Checklist for reviewer

• The code is easy to understand
• The code follows
  the coding guidelines of
  this project
• The code is tested or if not, the reason is documented or discussed
• The added and existing tests reasonably cover the code change
• The code has no breaking changes in the database schema or if so, the assignee is aware of it and
  will perform a database migration