Use input if default not set for parsing file name

[skliper]

Is your feature request related to a problem? Please describe.
CFE_FS_ParseInputFileName handler sets output to input as last resort (avoids uninit vars). CFE_FS_ParseInputFileNameEx only sets the output to default if default isn't null, so there's cases in the ES unit test where the output isn't initialized since there is no default value.

cFE/modules/core_api/ut-stubs/src/cfe_fs_handlers.c

Lines 171 to 176 in c161cf0

	if (status >= 0 && UT_Stub_CopyToLocal(UT_KEY(CFE_FS_ParseInputFileNameEx), OutputBuffer, OutputBufSize) == 0 &&
	OutputBufSize > 0 && DefaultInput != NULL)
	{
	/* Otherwise fall back to simple copy */
	strncpy(OutputBuffer, DefaultInput, OutputBufSize);
	}

Describe the solution you'd like
If DefaultInput is NULL, use InputName

Describe alternatives you've considered
None

Additional context
Any unit that uses CFE_FS_ParseInputFileNameEx with a NULL default risks uninit var, example:

cFE/modules/es/fsw/src/cfe_es_task.c

Lines 879 to 882 in c161cf0

	Result = CFE_FS_ParseInputFileNameEx(StartParams.BasicInfo.FileName, cmd->AppFileName,
	sizeof(StartParams.BasicInfo.FileName), sizeof(cmd->AppFileName), NULL,
	CFE_FS_GetDefaultMountPoint(CFE_FS_FileCategory_DYNAMIC_MODULE),
	CFE_FS_GetDefaultExtension(CFE_FS_FileCategory_DYNAMIC_MODULE));

Requester Info
Jacob Hageman - NASA/GSFC