Expression index can get an empty generated column name

If you CREATE INDEX on an expression without naming the index, Postgres
generates a name for the index column from that expression. For some
expressions the generated name comes out empty, so the index column is
left with an empty attname.

Example (a whole-row reference):

CREATE TABLE t (a int, b int);
CREATE INDEX ON t ((t IS NOT NULL));

SELECT attname, length(attname)
FROM pg_attribute
WHERE attrelid = 't__idx'::regclass AND attnum = 1;
attname | length
---------+--------
| 0
(1 row)

The index is named "t__idx" -- the doubled underscore is where the
empty column name landed.

Cause: ChooseIndexExpressionName() builds the name from the Vars,
Consts, and function names in the expression. A whole-row Var is
skipped and a punctuation-only Const is stripped away, so such
expressions contribute no text and the result is empty. Before commit
181b6185c7, expression columns were always named "expr" (giving
"t_expr_idx" here).

Fix (attached): if the walk finds nothing usable, fall back to "expr".
Ordinary expressions are unaffected. Includes a regression test; make
check is green. Against the master.

Patch is attached.

Thanks,
Dhruv Chauhan