Fix reading series from cache if fetching fails

`Fetcher::getByUri()` returns a string (and since it is a public method
we don't want to change that for now), but we check for `false`; so we
change that.

We also use `file_exists()` instead of `Cache::isFileCached()` because
the method is actually called `Cache::fileIsCached()` but appears to be
broken due to applying `::getCacheablePath()` again.  Furthermore the
other methods of `Cache` also use `file_exists()` directly.  Since
`Cache::fileIsCached()` is public, we do not remove the otherwise
unused method yet.

Author: cmb69

lib/php/libsdk/SDK/Cache.php

@@ -79,7 +79,7 @@ public function getCachedContent(string $path, bool $relative = false) : ?string
 	{/*{{{*/
 		$p = $this->getCacheablePath($path, $relative);
 
-		if ($this->isFileCached($p)) {
+		if (file_exists($p)) {
 			return file_get_contents($p);
 		}
 

lib/php/libsdk/SDK/Config.php

@@ -146,7 +146,7 @@ public static function getKnownBranches() : array
 			$fetcher = new Fetcher(self::$depsHost, self::$depsPort, self::$depsUriScheme);
 
 			$tmp = $fetcher->getByUri(self::$depsBaseUri . "/series/");
-			if (false !== $tmp) {
+			if ("" !== $tmp) {
 				$data = array();
 				if (preg_match_all(",packages-(.+)-(v[cs]\d+)-(x86|x64|arm64)-(stable|staging)\.txt,Us", $tmp, $m, PREG_SET_ORDER)) {
 					foreach ($m as $b) {