Material-ui: Shrink InputLabel wrong css when displayEmpty with Select outlined

[thaind97git]

When I set InputLabel with shrink={true} and FormControl is outlined, The InputLabel background is not correct

• [X] The issue is present in the latest release.
• [X] I have searched the issues of this repository and believe that this is not a duplicate.

Current Behavior 😯

The background of InputLabel (shrink={true}) when use FormControl (variant="outlined") is transparent ( not 'white' as expected )

Expected Behavior 🤔

When I use InputLabel ( shrink={true} ) when use FormControl ( variant="outlined" ), the background of InputLabel shoud be 'white'

Steps to Reproduce 🕹

Steps:
Link Codesandbox: https://codesandbox.io/s/material-demo-4dtlw

1. Choose default option
2. Lose focus to the select
3. The InputLabel background is transparent ( not 'white')

Context 🔦

I trying to use shrink={true} and variant="outlined" with the Select, but the css is not correct.

Your Environment 🌎

| Tech | Version |
| ----------- | ------- |
| Material-UI | v4.10.0 |
| React | v16.8.6 |
| Browser | |

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this issue appears to be a support request.

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[oliviertassinari]

@thaind97git You need to tell the select that the notch should be "open".

[malyzeli]

After a few hours of frustration I finally found the (hopefully correct) solution:

<FormControl variant="outlined">
  <InputLabel shrink>{label}</InputLabel>
  <Select input={<OutlinedInput notched label={label} />} />
</FormControl>

I suggest adding such an example to the documentation, considering I found dozens of issues and/or questions related to the same problem, with no actual working answer though.

This is really confusing especially for someone who just started using material-ui - why it's needed to pass label prop explicitly to OutlinedInput, when we already passed it to FormControl and InputLabel (and it didn't change a thing)...