Fix null reference in metadata

src/lib/Metadata.vala

@@ -14,6 +14,10 @@ namespace Ppub {
 
         private Gee.HashMap<string, string> entries {get; set;}
 
+        public Metadata() {
+            entries = new Gee.HashMap<string, string>();
+        }
+
         public string? get_value(string name) {
             if(entries.has_key(name)) {
                 return entries.get(name);