SelectWidget is lacking placeholder

[adrianparvino]

Prerequisites

• [X] I have read the documentation;
• [ ] In the case of a bug report, I understand that providing a SSCCE example is tremendously useful to the maintainers.
• [ ] Ideally, I'm providing a sample JSFiddle or a shared playground link demonstrating the issue.

Description

The core package uses placeholder as one of the options[1]. This option was not included in the mui theme [2](Find in page "placeholder"), and consequently was also not included in this theme.

[1] https://github.com/rjsf-team/react-jsonschema-form/blob/master/packages/core/src/components/widgets/SelectWidget.js#L94-L96 [2] https://github.com/rjsf-team/react-jsonschema-form/blob/master/packages/material-ui/src/SelectWidget/SelectWidget.tsx

[anikethsaha]

Fixed