sql: improve type checking error messages with AnnotateTypeExpr of NULL

[mgartner]

The expression NULL:::T (a NULL value annotated as type T) can lead to confusing error messages. This is because when the expression is type-checked, a tree.DNull value is returned, which includes no information about the annotated type T - the resolved type of tree.DNull is always types.Unknown. An expression that contains an annotated NULL and fails type checking often has a confusing error message as a result.

For example, notice that the error message below states "found type unknown", when "found type int" would make more sense:

defaultdb> SELECT CASE WHEN true THEN ('a', 2) ELSE NULL:::INT END;
ERROR: incompatible value type: expected NULL:::INT8 to be of type tuple, found type unknown

Epic CRDB-2474
Jira issue: CRDB-14073