Fix USB drive detection logic for empty arrays

Replaces null comparison with count check to properly handle empty arrays. The previous condition would incorrectly pass when the array exists but contains no elements, potentially causing the function to proceed without a valid USB drive.

FFUDevelopment/BuildFFUVM.ps1

@@ -3494,7 +3494,7 @@ Function Get-USBDrive {
     }
     
     # Check if any USB drives were found
-    if ($null -eq $USBDrives) {
+    if ($USBDrives.Count -eq 0) {
         WriteLog "No USB drive found. Exiting"
         Write-Error "No USB drive found. Exiting"
         exit 1