Fix null passed to strlen in parsers

When a command is run and fails it returns null.
This is then passed to the parser and it handles it as an empty string

Since PHP 8.0 it is deprecated to pass null to `strlen` so instead make
this an empty String.

Author: Patrick-Beuks

src/Gitonomy/Git/Parser/ParserBase.php

@@ -25,7 +25,7 @@ abstract protected function doParse();
     public function parse($content)
     {
         $this->cursor = 0;
-        $this->content = $content;
+        $this->content = $content ?? '';
         $this->length = strlen($this->content);
 
         $this->doParse();

src/Gitonomy/Git/ReferenceBag.php

@@ -354,7 +354,7 @@ protected function initialize()
             $parser = new Parser\ReferenceParser();
             $output = $this->repository->run('show-ref');
         } catch (RuntimeException $e) {
-            $output = null;
+            return;
         }
         $parser->parse($output);
 

tests/Gitonomy/Git/Tests/ReferenceBagTest.php

@@ -44,4 +44,13 @@ public function testUnknownReference(Repository $repository)
         $this->assertArrayNotHasKey('refs/pull/1/head', $refs);
         $this->assertArrayNotHasKey('refs/notes/gtm-data', $refs);
     }
+
+    /**
+     * @dataProvider provideEmpty
+     */
+    public function testEmptyRepo(Repository $repository)
+    {
+        $refs = $repository->getReferences()->getAll();
+        $this->assertSame([], $refs);
+    }
 }