fix 'static_url_path' defaulting for empty paths

-prefix a path delimiter iff there's a path to delimit
-ensures a valid default static route rule is created on application
intialisation for the case 'static_folder=""' and implicit
'static_url_path'

tests/test_basic.py

@@ -1427,6 +1427,20 @@ def test_static_url_path_with_ending_slash():
         assert flask.url_for("static", filename="index.html") == "/foo/index.html"
 
 
+def test_static_url_null_path(app):
+    app = flask.Flask(__name__, static_folder='', static_url_path='')
+    rv = app.test_client().open('/static/index.html', method='GET')
+    assert rv.status_code == 200
+    rv.close()
+
+
+def test_static_url_null_path_defaulting(app):
+    app = flask.Flask(__name__, static_folder='')
+    rv = app.test_client().open('/static/index.html', method='GET')
+    assert rv.status_code == 200
+    rv.close()
+
+
 def test_static_route_with_host_matching():
     app = flask.Flask(__name__, host_matching=True, static_host="example.com")
     c = app.test_client()