[ExpressionLanguage] made minor tweaks

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #9087
| License       | MIT
| Doc PR        | n/a

 * made ExpressionLanguage::parse return an ParsedExpression instance
 * optimized the size of the serialization of expressions