fix(components): [space] missing empty string size

[HeftyKoo]

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🧪 Playground Preview: https://element-plus.run/?pr=8039
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[jw-foss]

Syntactically I don't think we need this maybe adding fallbacks to falsy value might be a better solution?

[tolking]

This is indeed a ts syntax error, maybe the components size should not accept '' value.

[HeftyKoo]

Syntactically I don't think we need this maybe adding fallbacks to falsy value might be a better solution?

It's more like a typo issue, may be exclude the empty string is better.