patch turn nan to mask in target image

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 66.66667% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 79.43%. Comparing base (1136011) to head (6ea4193).

Files	Patch %	Lines
astrophot/plots/image.py	57.14%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #160 +/- ## ========================================== + Coverage 79.41% 79.43% +0.02% ========================================== Files 90 90 Lines 7665 7669 +4 ========================================== + Hits 6087 6092 +5 + Misses 1578 1577 -1 ```

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