Fix mount for data source with empty credentials schema

[naspeh]

When I was checking the hn example I got an error

root@b809fd2dba0c:/# sgr mount hackernews hackernews
error: KeyError: 'properties'

root@b809fd2dba0c:/# sgr -v DEBUG mount hackernews hackernews
error: Traceback (most recent call last):
error:   File "/usr/local/lib/python3.8/site-packages/splitgraph/commandline/__init__.py", line 115, in invoke
error:     result = super(click.Group, self).invoke(ctx)
error:   File "/usr/local/lib/python3.8/site-packages/click/core.py", line 1259, in invoke
error:     return _process_result(sub_ctx.command.invoke(sub_ctx))
error:   File "/usr/local/lib/python3.8/site-packages/click/core.py", line 1259, in invoke
error:     return _process_result(sub_ctx.command.invoke(sub_ctx))
error:   File "/usr/local/lib/python3.8/site-packages/click/core.py", line 1066, in invoke
error:     return ctx.invoke(self.callback, **ctx.params)
error:   File "/usr/local/lib/python3.8/site-packages/click/core.py", line 610, in invoke
error:     return callback(*args, **kwargs)
error:   File "/usr/local/lib/python3.8/site-packages/splitgraph/commandline/mount.py", line 69, in _callback
error:     mount(schema, mount_handler=handler_name, handler_kwargs=handler_options)
error:   File "/usr/local/lib/python3.8/site-packages/splitgraph/hooks/mount_handlers.py", line 68, in mount
error:     source = data_source.from_commandline(engine, handler_kwargs)
error:   File "/hn_fdw/hn_fdw/mount.py", line 42, in from_commandline
error:     self = super().from_commandline(*args, **kwargs)
error:   File "/usr/local/lib/python3.8/site-packages/splitgraph/hooks/data_source/fdw.py", line 71, in from_commandline
error:     for k in cast(Dict[str, Any], cls.credentials_schema["properties"]).keys():
error: KeyError: 'properties'

I was thinking about just fixing HackerNewsDataSource like this:

--- a/examples/custom_fdw/src/hn_fdw/mount.py
+++ b/examples/custom_fdw/src/hn_fdw/mount.py
@@ -34,7 +34,7 @@ _all_endpoints = [
 
 
 class HackerNewsDataSource(ForeignDataWrapperDataSource):
-    credentials_schema = {"type": "object"}
+    credentials_schema = {"type": "object", "properties": {}}
 
     @classmethod
     def from_commandline(cls, *args, **kwargs):

But decided to go a little bit further and create a test reproducing error and more general fix.

[mildbyte]

Good catch, thanks! I'll merge this so that CI/typechecks can run on our side.