[pigment-css] Fix keyframes serialize styles error

[siriwatknp]

Root cause

Passing empty args ([]) to emotion's serializeStyles is not allowed.

Full error:

1) Pigment CSS - keyframes
       basics:
     Expected test not to call console.error() but instead received 1 calls.
If the warning is expected, test for it explicitly by using the toErrorDev() matcher.
Test location:
  /Users/siriwatknp/Personal-Repos/material-ui/packages/pigment-react/tests/keyframes/keyframes.test.ts 
console.error message #1:
  You have illegal escape sequence in your template literal, most likely inside content's property value.
Because you write your CSS inside a JavaScript string you actually have to do double escaping, so for example "content: '\00d7';" should become "content: '\\00d7';".
You can read more about this here:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Template_literals#ES2018_revision_of_illegal_escape_sequences
Stack:
    at Object.serializeStyles (/Users/siriwatknp/Personal-Repos/material-ui/node_modules/.pnpm/@[email protected]/node_modules/@emotion/serialize/dist/emotion-serialize.cjs.dev.js:278:17)
    at KeyframesProcessor.generateArtifacts (/Users/siriwatknp/Personal-Repos/material-ui/packages/pigment-react/src/processors/keyframes.ts:97:6)
    at KeyframesProcessor.handleTemplate (/Users/siriwatknp/Personal-Repos/material-ui/packages/pigment-react/src/processors/keyframes.ts:92:7)
    at KeyframesProcessor.build (/Users/siriwatknp/Personal-Repos/material-ui/packages/pigment-react/src/processors/keyframes.ts:51:5)

The rest of the changes are splitting tests into small cases

---

• I have followed (at least) the PR section of the contributing guide.

[mui-bot]

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Generated by 🚫 dangerJS against 5f36e63

[siriwatknp]

follow the styled implementation.

[siriwatknp]

This condition is required to prevent ILLEGAL_ESCAPE_SEQUENCE_ERROR from emotion because serializeStyles assumes that [] is an expression.