Let the user query issues without default project

When the user sets --project to an empty string, we will not add the
default project from the config anymore.

jiracli/cli.py

@@ -102,9 +102,8 @@ def version(client):
 @pass_client
 @pass_config
 def issues(config, client, project, components, status, labels, texts, flagged, order, asc, jql):
-    if not project and config.has_section('filters') and config.get('filters', 'default_project'):
+    if project is None and config.has_section('filters') and config.get('filters', 'default_project'):
         project = config['filters']['default_project']
-        project_jql = f"project = {project}"
 
     # TODO add filtering by created since
     # TODO add filtering by updated
@@ -145,6 +144,7 @@ def issues(config, client, project, components, status, labels, texts, flagged,
         filters.append(flagged_jql)
 
     if project:
+        project_jql = f"project = {project}"
         filters.append(project_jql)
 
     jql = ' AND '.join(filters)