[core] Preserve quoted empty and null keys/values in string-to-MAP cast

[wangyusheng1985]

Purpose

String-to-MAP casting split entries first and then used a regex to separate key from value, so quote state for the individual key or value was gone by the time parseValue ran. {"" -> a} lost the empty key and {k -> "null"} became a SQL NULL value.

This change splits entries and then key/value with TokenSplitter.splitRaw so quotes survive, then parses each side through TokenSplitter.split the same way ARRAY and ROW already do.

Fixes #9782

Tests

mvn -pl paimon-common -DwildcardSuites=none -Dtest=CastExecutorTest spotless:check test passed (46 tests). Coverage includes quoted empty keys/values, quoted vs unquoted null, escaped null, quotes around -> and ,, and the MAP() function form.

Validation observed for this change:

• mvn -pl paimon-common -DwildcardSuites=none -Dtest=CastExecutorTest spotless:check test

Fixes #9782