Change format string to avoid "‘%s’ directive argument is null error"

[thnkslprpt]

Checklist

• I reviewed the Contributing Guide.
• I performed a cursory search to see if the bug report is relevant, not redundant, nor in conflict with other tickets.

Describe the bug
Build + Run workflow fails due to GCC warning (treated as error) of potential NULL argument being passed to format string.
This issue was noticed/raised here in PR #61 (comment)

To Reproduce
Run CI workflow (specifically [Build and run with startup msg verification) and observe error.

Expected behavior
CI workflow run without failure.

Code snips

/home/runner/work/DS/DS/apps/ds/fsw/src/ds_file.c:881:13: error: ‘%s’ directive argument is null [-Werror=format-overflow=]
  881 |             CFE_EVS_SendEvent(DS_MOVE_FILE_ERR_EID, CFE_EVS_EventType_ERROR,
      |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  882 |                               "FILE MOVE error: dir name = '%s', filename = '%s'", PathName, FileName);

Reporter Info
Avi @thnkslprpt