Allow no route to be defined for only serving static files

[Neradoc]

Just removed the exception when no route has been setup.
Makes something like that work to serve a static website.

import os
import socketpool
import wifi
from adafruit_httpserver.server import HTTPServer

PORT = 80
ROOT = "/www"

wifi.radio.connect(os.getenv("WIFI_SSID"), os.getenv("WIFI_PASSWORD"))
pool = socketpool.SocketPool(wifi.radio)
server = HTTPServer(pool)

print(f"Listening on http://{wifi.radio.ipv4_address}:{PORT}")
server.serve_forever(str(wifi.radio.ipv4_address), port=PORT, root_path=ROOT)

[anecdata]

Intended to serve an index.html only, but not directory listings or other filenames?
(Valid paths return empty page)

edit: paths ending in full filename work fine

[Neradoc]

Without this PR, trying to access any page with the above code.py causes:

Traceback (most recent call last):
  File "code.py", line 15, in <module>
  File "adafruit_httpserver/server.py", line 77, in serve_forever
  File "adafruit_httpserver/server.py", line 162, in poll
  File "adafruit_httpserver/route.py", line 109, in find_handler
ValueError: No routes added

With the fix it serves all static files in root_path.
That avoids having to create a dummy route like:

@server.route("/")
def base(request: HTTPRequest):
    with HTTPResponse(request, content_type=MIMEType.TYPE_HTML) as response:
        response.send_file("index.html")

Basically there should be no need to declare a route to serve a static website.

For example the Reactle app used in the standalone http server guide works with that code.py and that PR. (Once placed in www)

[anecdata]

Definitely an improvement for index.html and other files. Deleted my other comments (probably some erroneous testing), I'll re-test properly and comment if there's anything odd.

edit: 404 on a valid directory is the behavior pre-PR too

[anecdata]

LGTM!