fix(postgres)!: Parse empty bracketed ARRAY with cast

[VaggelisD]

Fixes #4674

• AST before:

>>> ast = sqlglot.parse_one("SELECT ARRAY[]::INT[]", dialect="postgres")
>>> ast
Select(
  expressions=[
    Cast(
      this=DataType(
        this=Type.ARRAY,
        expressions=[
          DataType(this=Type.ARRAY, nested=True)],
        nested=True),
      to=DataType(
        this=Type.ARRAY,
        expressions=[
          DataType(this=Type.INT, nested=False)],
        nested=True))])

• AST after

>>> ast = sqlglot.parse_one("SELECT ARRAY[]::INT[]", dialect="postgres")
>>> ast
Select(
  expressions=[
    Cast(
      this=Array(
        ),
      to=DataType(
        this=Type.ARRAY,
        expressions=[
          DataType(this=Type.INT, nested=False)],
        nested=True))])

Note how before, the ARRAY[] would be confused into nesting itself even though on Postgres it's a 1-D array:

psql> SELECT ARRAY[1]::INT[];
 array
-------
 {1}
(1 row)