fix(#2651): when scatter data length = 1 and x is cat

[hustcc]

PR includes

• fixed 🐛 [BUG]g2plot 2.x 气泡图 ，当数据只有一条时，并且 x轴的 type === cat 时，不展示，数据两条及以上就可以显示 #2651
• add / modify test cases
• documents, demos

Screenshot

Before	After
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[hustcc]

1. transformOption 和 getMeta 功能重复
2. scatter.type 错误
3. getMeta 方法简单重构（善用解构语法）

[github-actions]

🎊 PR Preview eb8892c has been successfully built and deployed to https://antvis-G2Plot-preview-pr-2661.surge.sh?type=diff&date=2021-06-29

🕐 Build time: 86.062s

_{🤖 By surge-preview}

[coveralls]

Pull Request Test Coverage Report for Build 978343965

• 7 of 7 (100.0%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.03%) to 95.974%

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Files with Coverage Reduction	New Missed Lines	%
src/utils/transform/word-cloud.ts	1	97.75%

Totals
Change from base Build 968502287:	-0.03%
Covered Lines:	6027
Relevant Lines:	6124

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💛 - Coveralls