fix 'static_url_path' defaulting for empty paths

[elbeardmorez]

the documentation of the application object specifies static parameters:

static_url_path – can be used to specify a different path for the static
files on the web. Defaults to the name of the static_folder folder.

static_folder – the folder with static files that should be served at
static_url_path. Defaults to the 'static' folder in the root path of the
application.

so by my understanding,

app=Flask(__name__, static_folder='')

should allow for arbitrary (absolute) path structure for all static
files. it doesn't. but the following does:

app=Flask(__name__, static_folder='', static_url_path='')

given the above doc strings, that appears inconsistent

for this specific case (static_folder=''), stepping through the
codebase yielded:

<Rule '//' (GET, OPTIONS, HEAD) -> static>
re.compile('^\\|//(?P<filename>[^/].*?)$')

which is an expression that can never be satisfied by a valid url

please consider this trivial patch. i hope the tests are appropriate
too, it was difficult to know how much of the mechanism to incorporate
for such a small change

commit message:

-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'

todo

• add a changelog entry if this patch changes code

[davidism]

It looks like this was addressed in a different way by #3182, but this solution seems more correct.