ZSQLCatalog.SearchText: Fix parsing of COLUMN OPERATOR string

...when COLUMN is not a valid column (hence mutated to a STRING_PREFIX
token at lexer level).
Kudos to Tristan for triggering this bug.

product/ZSQLCatalog/SearchText/AdvancedSearchTextParser.py

@@ -275,9 +275,10 @@ class AdvancedSearchTextParser(lexer):
   def p_string(self, p):
     '''string : WORD
               | STRING
-              | STRING_PREFIX string'''
-    if len(p) == 3:
-      p[0] = p[1] + p[2]
+              | STRING_PREFIX string
+              | STRING_PREFIX OPERATOR string'''
+    if len(p) > 2:
+      p[0] = ''.join(p[1:])
     else:
       p[0] = p[1]