fix dynamic_source_filename return for contracts without filename

[cfcfs]

What I did

Fix coverage report failing with Plugin 'boa.coverage.TitanoboaPlugin' did not provide a file reporter for 'None'. Checking with coverage debug data shows a line with

None: 9 lines [boa.coverage.TitanoboaPlugin]

How I did it

Change TitanoboaTracer.dynamic_source_filename to return None if there's no contract filename (previously it returned "None")

How to verify it

Description for the changelog

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[charles-cooper]

thanks!