Empty css module block should not be ignored

[Astray-git]

Version

15.7.0

Reproduction link

https://codesandbox.io/s/vmlxvlwo4l

Steps to reproduce

1. edit example/source.vue with an empty css module block
2. yarn dev

What is expected?

The text "hello" rendered correctly ( as before 15.5.1 ). with warnings in console.
Or, emit a compile error about empty css module block

What is actually happening?

Compiled successfully.
Runtime error in page: Cannot read property 'red' of undefined

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Related: #1464 #1496

Possible solutions:

1. emit error on empty css module block.
2. generate empty normally ( as before 15.5.1 ), then emit warning.