Fix contrib/pg_trgm's similarity() function for trigram-free strings.

Cases such as similarity('', '') produced a NaN result due to computing
0/0.  Per discussion, make it return zero instead.

This appears to be the basic cause of bug #7867 from Michele Baravalle,
although it remains unclear why her installation doesn't think Cyrillic
letters are letters.

Back-patch to all active branches.

contrib/pg_trgm/expected/pg_trgm.out

@@ -53,6 +53,12 @@ select similarity('wow',' WOW ');
           1
 (1 row)
 
+select similarity('---', '####---');
+ similarity 
+------------
+          0
+(1 row)
+
 CREATE TABLE test_trgm(t text);
 \copy test_trgm from 'data/trgm.data
 select t,similarity(t,'qwertyu0988') as sml from test_trgm where t % 'qwertyu0988' order by sml desc, t;