fix(compiler-core): recognize empty string as non-identifier

[KazariEX]

https://play.vuejs.org/#eNp9kLuOwjAQRX8lmpoNxW6FIqTdFQUUgIDSTZQMweB4LHscIqH8O7YRjwLR2fecse74Ar/G5J1HmEDB2BpVMk6FzrKill3Wfe2J8mkxDpeQFuMXBUbAriK9l01+dKTDC5c4KKCi1kiFdmVYknYCJlkikZVK0XmRMrYeR/e8OmB1epMfXR8zAWuLDm2HAh6MS9sg3/Bsu8Q+nB+wpdqrYH+AG3SkfOx40/68rkPtFy+1nbeGLEvd7NysZ9TuvlQsGs0h+QLCN/5/WP1Z9zv/SXNCDzBcAU2TfbA=

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Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	100 kB	37.9 kB	34.2 kB
vue.global.prod.js	158 kB (+3 B)	57.8 kB (+4 B)	51.4 kB (+10 B)

Usages

Name	Size	Gzip	Brotli
createApp (CAPI only)	46.6 kB	18.3 kB	16.7 kB
createApp	54.6 kB	21.3 kB	19.4 kB
createSSRApp	58.7 kB	23 kB	20.9 kB
defineCustomElement	59.4 kB	22.8 kB	20.8 kB
overall	68.4 kB	26.4 kB	24 kB

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commit: c9baf56

[skirtles-code]

Is v-foo. considered valid template syntax? Maybe it should be a compiler error?

Should this PR include a test?

[KazariEX]

I currently have no opinion on whether an empty string should be allowed in modifiers.

[KazariEX]

We can address the issue of warning on empty strings as arg or modifiers in another PR.