Allow empty values in FileBrowseField's with blank=True, null=False

filebrowser.diff

diff --git a/filebrowser/fields.py b/filebrowser/fields.py
--- a/filebrowser/fields.py
+++ b/filebrowser/fields.py
@@ -96,10 +96,12 @@
         return FileObject(url_to_path(value))

     def get_db_prep_value(self, value, connection, prepared=False):
         if value is None:
             return None
+        if not value:
+            return value
         return value.url_save

     def formfield(self, **kwargs):
         attrs = {}
         attrs["directory"] = self.directory