Fix #63, Change format string to avoid GCC workflow failure

[thnkslprpt]

Checklist

• I reviewed the Contributing Guide.
• I signed and emailed the appropriate Contributor License Agreement to [email protected] and copied [email protected].

Describe the contribution
Fixes #63
Format string in CFE_EVS_SendEvent instance changed to avoid potential NULL argument being passed in to "%s" format specifier, which causes a GCC error and workflow failure.

Testing performed
GitHub CI actions (incl. Build + Run, Unit Tests etc.) all passing successfully.

Expected behavior changes
Workflow (Build + Run) will complete without GCC error causing failure.

Contributor Info
Avi @thnkslprpt