seedlink: fix a bug in basic client get_info()

[megies]

What does this PR do?

Handles a bug in seedlink basic client get_info(), first reported in #2808

Channels for stations without data made it into the info results and caused an exception due to fnmatch chocking on None values. No-data stations will now be removed from results and a warning is shown indicating what stations were removed (with a kwarg option to suppress the warning)

Why was it initiated? Any relevant Issues?

closes #2808

PR Checklist

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[megies]

Failures unrelated