fix: crash when leaving out system parameter

The `--system` parameter can be supplied via command line
or selected afterwards.

However if none was provided the command would crash with
`TypeError: Cannot assign null to property $system`.

Handle that gracefully and make the type spec more precise
for the setSystem function.

Signed-off-by: Max <max@nextcloud.com>

lib/Service/Importer/BoardImportService.php

@@ -143,11 +143,13 @@ public function validateSystem(): void {
 	}
 
 	/**
-	 * @param mixed $system
+	 * @param ?string $system
 	 * @return self
 	 */
 	public function setSystem($system): self {
-		$this->system = $system;
+		if ($system) {
+			$this->system = $system;
+		}
 		return $this;
 	}