is_empty for OsStr

[ticki]

Part of #30520

[rust-highfive]

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[ticki]

fixes #30259

[ticki]

r? @gankro

[Gankra]

Travis seems unhappy

[ticki]

It compiles on my machine, and the tests runs fine. I don't understand why travis is sad. I'm also unable to find travis's complains in the travis log.

[Gankra]

failures:

---- path::Path::is_empty_0 stdout ----
    <anon>:8:19: 8:29 error: use of unstable library feature 'os_extras': recently added (see issue #30259)
<anon>:8     assert!(!path.is_empty());
                           ^~~~~~~~~~
<anon>:8:5: 8:31 note: in this expansion of assert! (defined in <std macros>)
<anon>:8:19: 8:29 help: add #![feature(os_extras)] to the crate attributes to enable
error: aborting due to previous error
thread 'path::Path::is_empty_0' panicked at 'Box<Any>', src/libsyntax/errors/mod.rs:269


failures:
    path::Path::is_empty_0

test result: �[31mFAILED�(B�[0m. 443 passed; 1 failed; 11 ignored; 0 measured

[frewsxcv]

You need to add a #![feature(os_extras)] here in this code block

[alexcrichton]

Closing due to inactivity, but feel free to resubmit with tests passing!

[frewsxcv]

Rebased this and fixed in #31231

[ticki]

Thanks @frewsxcv!