fix: Make sure getNumConcurrentPreviews never returns 0

Tests were stuck forever waiting for a semaphore because concurrency was
 returning 0. It was previously working by chance because cache of the
 value in a static var was leaking through tests.

Signed-off-by: Côme Chilliet <come.chilliet@nextcloud.com>

Author: come-nc

lib/private/Preview/Generator.php

@@ -306,6 +306,9 @@ public function getNumConcurrentPreviews(string $type): int {
 			default:
 				return -1;
 		}
+		if ($this->cachedNumConcurrentPreviews[$type] < 1) {
+			$this->cachedNumConcurrentPreviews[$type] = 1;
+		}
 		return $this->cachedNumConcurrentPreviews[$type];
 	}