[SPARK-4268][SQL] Use #::: to get benefit from Stream in SqlLexical.allCaseVersions

In addition, using `s.isEmpty` to eliminate the string comparison.

Author: zsxwing <zsxwing@gmail.com>

Closes #3132 from zsxwing/SPARK-4268 and squashes the following commits:

358e235 [zsxwing] Improvement of allCaseVersions

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/SparkSQLParser.scala

@@ -97,10 +97,10 @@ class SqlLexical(val keywords: Seq[String]) extends StdLexical {
 
   /** Generate all variations of upper and lower case of a given string */
   def allCaseVersions(s: String, prefix: String = ""): Stream[String] = {
-    if (s == "") {
+    if (s.isEmpty) {
       Stream(prefix)
     } else {
-      allCaseVersions(s.tail, prefix + s.head.toLower) ++
+      allCaseVersions(s.tail, prefix + s.head.toLower) #:::
         allCaseVersions(s.tail, prefix + s.head.toUpper)
     }
   }