add isblank

[mulimoen]

Fixes #1509

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @gnzlbg (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[gnzlbg]

@bors: r+

[bors]

📌 Commit b11e2d4 has been approved by gnzlbg

[bors]

⌛ Testing commit b11e2d4 with merge ad350f2...

[bors]

💔 Test failed - status-azure

[mulimoen]

Seems the checks passed now!

[gnzlbg]

@bors: r+

[bors]

📌 Commit e7f1134 has been approved by gnzlbg

[bors]

⌛ Testing commit e7f1134 with merge 49c1d13...

[bors]

☀️ Test successful - checks-cirrus-freebsd-11, checks-cirrus-freebsd-12, status-azure
Approved by: gnzlbg
Pushing 49c1d13 to master...