fix: version imprint defaults to None for missing keys

[sebastianbrandhuber]

Changelog Description

Missing keys in version data imprint will not result in a key error anymore and are eventually removed from the data dictionary.

Additional review information

More information about this here

Fix #268

Testing notes:

1. Have a published gizmo where one of these attributes is missing:

            "frameStart",
            "frameEnd",
            "handleStart",
            "handleEnd",
            "source",
            "fps"

for example a custom LUT ingest where frameStart or frameEnd could not be determined.
3. Load the gizmo with the loader - should not fail.

[BigRoy]

LGTM - None values are skipped below.