placeholder in time-picker can be empty string

[jialeicui]

• Make sure you follow antd's code convention.
• Run npm run lint and fix those errors before submitting in order to keep consistent code style.
• Rebase before creating a PR to keep commit history clear.
• Add some descriptions and refer relative issues for you PR.

[mention-bot]

@jialeicui, thanks for your PR! By analyzing the history of the files in this pull request, we identified @benjycui, @afc163 and @waywardmonkeys to be potential reviewers.

[coveralls]

Coverage remained the same at 71.903% when pulling fc6e9a5 on jialeicui:master into 3ecc5f6 on ant-design:master.

[benjycui]

👍