[FlatButton] Fix label validation for 0 (number data-type)

[mrdivyansh]

• PR has tests / docs demo, and is linted.
• Commit and PR titles begin with [ComponentName], and are in imperative form: "[Component] Fix leaky abstraction".
• Description explains the issue / use-case resolved, and auto-closes the related issue(s)

FlatButton throw an error when label is zero (numeric data-type). This is special case, so we should handle it in validation function.

[oliviertassinari]

I'm wondering if we couldn't

• use the warning module to remove some of the code in production
• use React.PropTypes.node to improve this validation method.

[mrdivyansh]

@oliviertassinari

• I don't think any warning is needed here. I am wondering to know, warning message for what ? Component user has not done any mistake if he passed 0 as label.
• Actually, Label validation doesn't depends on single property. We need to check presence of anyone property out of children, label, and icon. So we can't.

[mrdivyansh]

@mbrookes @oliviertassinari Any update ?

[oliviertassinari]

FlatButton throw an error when label is zero (numeric data-type).

@mrdivyansh Actually, I'm having some doubt about this statement. Have a loot at #4346.

[mrdivyansh]

@oliviertassinari @mbrookes If PR is still not good, please could you do it by own. we need this fix on master branch as soon as possible.

[mbrookes]

Tested in this PR: #4346, specifically here: https://github.com/callemall/material-ui/pull/4346/files#diff-30eabceade84f961fe2a4f955be1f9b1R177

Please open an issue if you believe this is still broken in master.