Fix generated properties file NPE.

[ryantse]

• Adds a test case to verify that the written properties file can be read again.

Context

Accidentally introduced an NPE when reading an existing properties file and doing the comparison. Added a test case to ensure coverage of this case.

Contributor Checklist

• Added relevant integration or unit tests to verify the changes
• Update the Readme or any other documentation (including relevant Javadoc)
• Ensured that tests pass locally: mvn clean package
• Ensured that the code meets the current checkstyle coding style definition: mvn clean verify -Pcheckstyle -Dmaven.test.skip=true -B

[ryantse]

@TheSnoozer: Missed an edge case in my last pull request where setProperty cannot be passed a null. Instead, this change sets them to empty strings for the purposes of comparison. I've also added a test here to make sure that we test rereading the generated property file.

[TheSnoozer]

Thanks for the follow up and adding a test-case :-)
Looks reasonable for me.

[TheSnoozer]

Refs #511