Handle strict typing in Checker and fix tests

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>

lib/private/IntegrityCheck/Checker.php

@@ -333,7 +333,12 @@ class Checker {
 			return [];
 		}
 
-		$signatureData = json_decode($this->fileAccessHelper->file_get_contents($signaturePath), true);
+		$content = $this->fileAccessHelper->file_get_contents($signaturePath);
+		$signatureData = null;
+
+		if (\is_string($content)) {
+			$signatureData = json_decode($content, true);
+		}
 		if(!\is_array($signatureData)) {
 			throw new InvalidSignatureException('Signature data not found.');
 		}

tests/lib/IntegrityCheck/CheckerTest.php

@@ -58,6 +58,9 @@ class CheckerTest extends TestCase {
 		$this->cacheFactory = $this->createMock(ICacheFactory::class);
 		$this->appManager = $this->createMock(IAppManager::class);
 
+		$this->config->method('getAppValue')
+			->will($this->returnArgument(2));
+
 		$this->cacheFactory
 			->expects($this->any())
 			->method('createDistributed')