Update Prometheus.php

Author: ohmydevops

src/Log/Prometheus.php

@@ -122,12 +122,9 @@ public function onPhpbuEnd(End $event)
         $this->write('# HELP phpbu_backup_size The size of the last successful backup' . PHP_EOL);
         $this->write('# TYPE phpbu_backup_size gauge' . PHP_EOL);
         foreach ($this->backupStats as $backupName => $backupStats) {
-            if($this->backupStats[$backupName]['size'] = "") {
-                $this->backupStats[$backupName]['size'] = 0;
-            }
             $this->write(
                 'phpbu_backup_size{name="' . $backupName . '"} ' .
-                $this->backupStats[$backupName]['size'] .
+                $this->backupStats[$backupName]['size'] = " " ? 0 : $this->backupStats[$backupName]['size'] .
                 PHP_EOL
             );
         }