Trunk-6470: Add JUnit tests for OpenmrsUtil.getFileAsBytes()

[Jagdish-Singh-Bisht]

This PR adds unit tests for the method OpenmrsUtil.getFileAsBytes(File file).

The test suite covers the following scenarios:

1. Valid file → method returns correct byte array

2. Empty file → method returns zero-length array

3. Null input → method returns null (without throwing exception)

4. Non-existent file → method handles exception internally and returns null

The tests help ensure correct behavior of file reading operations and improve code coverage for OpenmrsUtil.

Issue Link

https://issues.openmrs.org/browse/TRUNK-6470

What was changed

• Added new JUnit test methods inside OpenmrsUtilTest.java

• Covered both positive and negative test scenarios

• Verified behavior using FileUtils and StandardCharsets

Testing Instructions

To run only these tests:

mvn -q -pl :openmrs-api -Dtest=OpenmrsUtilTest test

Checklist

• My IDE is using Java 21

• mvn clean install completes successfully

• Tests pass on local environment

• PR title follows OpenMRS naming conventions

Additional Notes

This PR does not modify production code.
It only improves test coverage for core utilities.

[Jagdish-Singh-Bisht]

Hello, mentors

This PR adds unit tests for OpenmrsUtil.getFileAsBytes() as part of TRUNK-6470.
All tests pass locally with Java 21.
Kindly review the changes and approve the CI workflows.

Thank you!