Ensure hub_url is configured before use

[berghaus]

This PR follows the discussion in discourse post 18215.

Problem: the configuration of BinderHub fails if hub_url is not set.

Solution: set a default value for the hub_url in the helm chart. Check that hub_url is set before use.

I tested the helm chart (as per the documentation), but am unsure how to test the change to binderhub_config.py properly.

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[manics]

You can just set the default to "" in https://github.com/jupyterhub/binderhub/blob/6b16f8d3888263be8b3a1ccbc8f63734b55627ec/binderhub/app.py#L512-L513 e.g. https://github.com/jupyterhub/binderhub/blob/6b16f8d3888263be8b3a1ccbc8f63734b55627ec/binderhub/app.py#L404-L417

[berghaus]

You can just set the default to "" in

https://github.com/jupyterhub/binderhub/blob/6b16f8d3888263be8b3a1ccbc8f63734b55627ec/binderhub/app.py#L512-L513

Unfortunately, that won't address the issue. The error is raised because an unset configuration value is retrieved, not the BinderHub parameter that the configuration value is meant to set:

https://github.com/jupyterhub/binderhub/blob/ddba2322d9a3f8d8d1a91e405f4ceec488071740/helm-chart/binderhub/files/binderhub_config.py#L58

I proved this to myself like so:

# Sample configurable:
from traitlets.config.configurable import Configurable
from traitlets.config import get_config
from traitlets import Int, Unicode
from urllib.parse import urlparse

class MyClass(Configurable):
    name = Unicode(config=True)
    number = Int(config=True)

# Create a configuration
c = get_config()
c.MyClass.number = 10
try:
    urlparse(c.MyClass.name)
except AttributeError:
    print("using config item produced error")

# construct class instance:
m = MyClass()
try:
    urlparse(m.name)
except AttributeError:
    print("using instane property produced error")

print("test done")

which produces:

using config item produced error
test done

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