fix: Return null on empty files/images

[bellini666]

Fix #453

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (e751a2d) 87.86% compared to head (9f46e8c) 87.86%.

Files	Patch %	Lines
strawberry_django/resolvers.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #466   +/-   ##
=======================================
  Coverage   87.86%   87.86%           
=======================================
  Files          37       37           
  Lines        3148     3157    +9     
=======================================
+ Hits         2766     2774    +8     
- Misses        382      383    +1     

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