Fix #2553, null value in vsnprintf

[avan989]

Fix #2553, fix test that was passing null value
into vsnprintf

Checklist (Please check before submitting)

• I reviewed the Contributing Guide.
• I signed and emailed the appropriate Contributor License Agreement to GSFC-SoftwareRelease@mail.nasa.gov and copied cfs-program@lists.nasa.gov.

Describe the contribution
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• Include explicitly what issue it addresses [e.g. Fixes #X]

Fixes #2553, address issue that occur when running with address sanitizer. One of the coverage test was trying to pass an
null value to force an return code of 0 or less to an vsnprintf. Passing an null value as an input is an undefine behavior but you can achieve the same result by passing in an empty string which will force the return code to be 0.

Testing performed
Steps taken to test the contribution:
add the following options:
add_compile_options(-fsanitize=address -g)
add_link_options(-fsanitize=address)

1. make ENABLE_UNIT_TESTS=true SIMULATION=native prep
2. make install
3. make test
4. make lcov

Expected behavior changes
A clear and concise description of how this contribution will change behavior and level of impact.

no more error when running with fsanitize.
Still 100% coverage.

System(s) tested on

• Hardware: [e.g. PC, SP0, MCP750]
• OS: [e.g. Ubuntu 18.04, RTEMS 4.11, VxWorks 6.9]
• Versions: [e.g. cFE 6.6, OSAL 4.2, PSP 1.3 for mcp750, any related apps or tools]

Additional context
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Third party code
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  Anh Van, GSFC