[coverage] Disable save button if there is nothing to save
[coverage] Disable save button if there is nothing to save

Fixed: chromium:1064206
Change-Id: I4d0ffb00077fab94bf6485d4baba6a46b101f81a
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2119689
Reviewed-by: Simon Zünd <38944668e9e2ca98ccd0275b53370f90fa55c113@chromium.org>
Commit-Queue: Sigurd Schneider <f04d242e109628a27a0f75fd49a87722bcf291fd@chromium.org>

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