[DiskCache] Fix WPE_DISK_CACHE_SIZE env parsing

1) Fix WPE_DISK_CACHE_SIZE evn parsing.
   Previously it was falling back to "1" default in all cases.
2) Disable disk cache if no valid number provided

Author: asurdej-comcast

Source/WebKit/Shared/CacheModel.cpp

@@ -183,7 +183,7 @@ uint64_t calculateURLCacheDiskCapacity(CacheModel cacheModel, uint64_t diskFreeS
         if (units != 1)
             value = value.substring(0, value.length()-1);
 
-        size_t size = size_t(parseInteger<uint64_t>(s).value_or(1) * units);
+        size_t size = size_t(parseInteger<uint64_t>(value).value_or(0) * units);
         urlCacheDiskCapacity = (unsigned long)(size);
     }