Clear chord label state rotation when unset

Adopt the Copilot review suggestion for state label rotation cleanup. A missing state-specific rotate now writes null into the text state so zrender restores the normal rotation instead of keeping an old override.

Constraint: zrender treats null state transform props as absent and restores normal state values.

Rejected: Copy normal rotation into every state | works visually but obscures the intended state fallback semantics.

Confidence: high

Scope-risk: narrow

Tested: npx jest --config test/ut/jest.config.cjs --runTestsByPath test/ut/spec/series/chord.test.ts --coverage=false

Tested: npx eslint --no-cache src/chart/chord/ChordPiece.ts test/ut/spec/series/chord.test.ts

Tested: npm run checktype