Improved Coverage for test_digital_image_processing.py

[csana004]

Describe your change:

improved test coverage for digital_image_processing/test_digital_image_processing.py from 3% to 47%

"Fixes #9943 "

• [ ] Add an algorithm?
• [ Y] Fix a bug or typo in an existing algorithm?
• [ ] Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• [ ] Documentation change?

Checklist:

• [ Y] I have read CONTRIBUTING.md.
• [ Y] This pull request is all my own work -- I have not plagiarized.
• [ Y] I know that pull requests will not be merged if they fail the automated tests.
• [ Y] This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• [ Y] All new Python files are placed inside an existing directory.
• [ Y] All filenames are in all lowercase characters with no spaces or dashes.
• [ Y] All functions and variable names follow Python naming conventions.
• [ Y] All function parameters and return values are annotated with Python type hints.
• [ Y] All functions have doctests that pass the automated testing.
• [ ] All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• [ ] If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".