Nested FEEL path expressions resolving as `null` with `Null` type

### Describe the Bug

As the main issue describes, the FEEL path expressions that cannot be resolved in the diagram are resolved as `null` with `Null` type which is misleading as they're not explicitly `null`.

Cf. https://github.com/bpmn-io/variable-resolver/pull/98#issuecomment-4086119361

### Actual Behavior

- Nested FEEL path expressions resolve as `null` with `Null` type due to absence of intermediate variables/properties like `=agent.foo.bar` resolving as `null` when `agent={ bar: {} }`.

### Expected Behavior

- Nested FEEL path expressions that cannot be explicitly resolved to a certain value should be resolved as its FEEL expression with `Any` type.

### Additional context

I understand that it's not always the case that `=agent.foo.bar` should be resolved as `=agent.foo.bar` with Any type as it may be just misused or there's no element (a job worker or any other elements that overwrites the variable) populating the "absent" variables/properties. In those cases, we may want to actually resolve to `null` with `Null` type. However, this may be more relevant as of https://github.com/camunda/camunda-modeler/issues/5697.