BUGFIX: Support empty data in SimpleFileBackend

In newer versions of PHP, calling `fread` with non-positive length
raises an error:
fread(): Argument #2 ($length) must be greater than 0

Since SimpleFileBackend supports saving an empty file, it should also
support reading one.

Neos.Cache/Classes/Backend/SimpleFileBackend.php

@@ -503,7 +503,16 @@ protected function readCacheFile(string $cacheEntryPathAndFilename, int $offset
                     if ($offset !== null) {
                         fseek($file, $offset);
                     }
-                    $data = fread($file, $maxlen !== null ? $maxlen : filesize($cacheEntryPathAndFilename) - (int)$offset);
+
+                    $length = $maxlen !== null ? $maxlen : filesize($cacheEntryPathAndFilename) - (int)$offset;
+
+                    // fread requires a positive length. If the file is empty, we just return an empty string.
+                    if ($length === 0) {
+                        $data = '';
+                    } else {
+                        $data = fread($file, $length);
+                    }
+
                     flock($file, LOCK_UN);
                 }
                 fclose($file);

Neos.Cache/Tests/Unit/Backend/SimpleFileBackendTest.php

@@ -222,6 +222,22 @@ public function getReturnsContentOfTheCorrectCacheFile()
         self::assertSame($data2, $simpleFileBackend->get($entryIdentifier));
     }
 
+    /**
+     * @test
+     */
+    public function getSupportsEmptyData()
+    {
+        $this->mockCacheFrontend->expects(self::any())->method('getIdentifier')->will(self::returnValue('UnitTestCache'));
+
+        $data = '';
+        $entryIdentifier = 'SimpleFileBackendTest';
+
+        $simpleFileBackend = $this->getSimpleFileBackend();
+        $simpleFileBackend->set($entryIdentifier, $data);
+
+        self::assertSame($data, $simpleFileBackend->get($entryIdentifier));
+    }
+
     /**
      * @test
      */