Add null pointer check to CFE_ES_ReloadApp functional test

[skliper]

Is your feature request related to a problem? Please describe.
AppFIleName NULL not checked (or tested)

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

Lines 220 to 226 in e80aae9

	int32 CFE_ES_ReloadApp(CFE_ES_AppId_t AppID, const char *AppFileName)
	{
	int32 ReturnCode = CFE_SUCCESS;
	os_fstat_t FileStatus;
	CFE_ES_AppRecord_t *AppRecPtr = CFE_ES_LocateAppRecordByID(AppID);
	if (AppRecPtr == NULL)

Describe the solution you'd like
Add a NULL test, add the check (if needed)

Describe alternatives you've considered
None

Additional context
Code review

Requester Info
Jacob Hageman - NASA/GSFC

[zanzaben]

AppFileName is tested for null inside OS_stat which is the first place to use it.

[skliper]

Switched to unit-test only, since just the functional check is needed for the NULL pointer.

[zanzaben]

This will be covered by #806