[SPARK-43841][SQL] Handle candidate attributes with no prefix in StringUtils#orderSuggestedIdentifiersBySimilarity

[bersprockets]

What changes were proposed in this pull request?

In StringUtils#orderSuggestedIdentifiersBySimilarity, handle the case where the candidate attributes have a mix of empty and non-empty prefixes.

Why are the changes needed?

The following query throws a StringIndexOutOfBoundsException:

with v1 as (
 select * from values (1, 2) as (c1, c2)
),
v2 as (
  select * from values (2, 3) as (c1, c2)
)
select v1.c1, v1.c2, v2.c1, v2.c2, b
from v1
full outer join v2
using (c1);

The query should fail anyway, since b refers to a non-existent column. But it should fail with a helpful error message, not with a StringIndexOutOfBoundsException.

StringUtils#orderSuggestedIdentifiersBySimilarity assumes that a list of suggested attributes with a mix of prefixes will never have an attribute name with an empty prefix. But in this case it does (c1 from the coalesce has no prefix, since it is not associated with any relation or subquery):

+- 'Project [c1#5, c2#6, c1#7, c2#8, 'b]
   +- Project [coalesce(c1#5, c1#7) AS c1#9, c2#6, c2#8] <== c1#9 has no prefix, unlike c2#6 (v1.c2) or c2#8 (v2.c2)
      +- Join FullOuter, (c1#5 = c1#7)
         :- SubqueryAlias v1
         :  +- CTERelationRef 0, true, [c1#5, c2#6]
         +- SubqueryAlias v2
            +- CTERelationRef 1, true, [c1#7, c2#8]

Because of this, orderSuggestedIdentifiersBySimilarity returns a sorted list of suggestions like this:

ArrayBuffer(.c1, v1.c2, v2.c2)

UnresolvedAttribute.parseAttributeName chokes on an attribute name that starts with a '.'.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

New unit tests.

[HyukjinKwon]

cc @rednaxelafx @cloud-fan FYI

[MaxGekk]

+1, LGTM. Merging to master.
Thank you, @bersprockets and @dongjoon-hyun @HyukjinKwon for review.