[dashboard] metadata editing modal generates invalid dashboard url

[graceguo-supercat]

Related to #8876.

Expected results

Empty slug and save should work.

Actual results

Environment

(please complete the following information):

• superset version: latest master

Checklist

Make sure these boxes are checked before submitting your issue - thank you!

• I have checked the superset logs for python stacktraces and included it here as text if there are any.
• I have reproduced the issue with at least the latest released version of superset.
• I have checked the issue tracker for the same issue and I haven't found one similar.

Additional context

Add any other context about the problem here.

cc @suddjian

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[suddjian]

😱thanks for catching!