[CX] create array index results in misleading message when column type is not specified

The failing command is

create index uarra_idx_KS1 on KS1(UNNEST uarr_i) EXCLUDE UNKNOWN KEY;

Note that the type of the column uarr_i is not specified.

The error message obtained is

msg": "Cannot find datatype with name null (in line 1, at column 34)"