refs #69677: use metaData only if its a valid object if a .cache file is empty 0 bytes so the shop can be accessed again

If a .cache file is empty 0 bytes, the shop cannot be accessed

Can be reproduced as follows:

1. install store with G-Protector
2. call the store
3. open the file meta_data.cache on the FP server in the folder /GProtector/cache/ and remove the content
4. upload the file again
5. open the store again

Expected result: Store should still be callable and the file should be automatically filled with content again

Actual result: Store is no longer callable and an error message appears

Fatal error: Uncaught Error: Call to a member function modificationDateUnixTime() on boolean in /GProtector/classes/FilterCache.php:337 Stack trace: #0 /GProtector/classes/FilterCache.php(183): GProtector\FilterCache->getModificationDateUnixTime() 
#1 /GProtector/classes/FilterCache.php(84): GProtector\FilterCache->isCacheOlderThanRemoteFile() 
#2 /GProtector/classes/GProtector.inc.php(60): GProtector\FilterCache->renew() 
#3 /GProtector/start.inc.php(41): GProtector\GProtector->start() 
#4 /GXMainComponents/Application.inc.php(513): require('/var/www/tt/bet...') 
#5 /GXMainComponents/Application.inc.php(122): Gambio\GX\Application->runGProtector() 
#6 /includes/application_top.php(15): Gambio\GX\Application->run() 
#7 /index.php(27): include('/var/www/tt/bet...') 
#8 {main} thrown in /GProtector/classes/FilterCache.php on line 337