Add test case for group value is null

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/regexpExpressions.scala

@@ -555,7 +555,7 @@ case class RegExpExtract(subject: Expression, regexp: Expression, idx: Expressio
           There is a SQL config 'spark.sql.parser.escapedStringLiterals' that can be used to
           fallback to the Spark 1.6 behavior regarding string literal parsing. For example,
           if the config is enabled, the `regexp` that can match "\abc" is "^\abc$".
-      * idx - an int expression of the regex group index. The regex maybe contains multiple
+      * idx - an int expression of the regex group index. The regex may contains multiple
           groups. `idx` indicates which regex group to extract.
   """,
   examples = """
@@ -571,7 +571,6 @@ case class RegExpExtractAll(subject: Expression, regexp: Expression, idx: Expres
   override def nullSafeEval(s: Any, p: Any, r: Any): Any = {
     val m = getLastMatcher(s, p)
     val matchResults = new ArrayBuffer[UTF8String]()
-    val mr: MatchResult = m.toMatchResult
     while(m.find) {
       val mr: MatchResult = m.toMatchResult
       val index = r.asInstanceOf[Int]

sql/core/src/test/scala/org/apache/spark/sql/StringFunctionsSuite.scala

@@ -124,6 +124,8 @@ class StringFunctionsSuite extends QueryTest with SharedSparkSession {
 
   test("non-matching optional group") {
     val df = Seq(Tuple1("aaaac")).toDF("s")
+
+    // regexp_extract
     checkAnswer(
       df.select(regexp_extract($"s", "(foo)", 1)),
       Row("")
@@ -132,6 +134,16 @@ class StringFunctionsSuite extends QueryTest with SharedSparkSession {
       df.select(regexp_extract($"s", "(a+)(b)?(c)", 2)),
       Row("")
     )
+
+    // regexp_extract_all
+    checkAnswer(
+      df.select(regexp_extract_all($"s", "(foo)", 1)),
+      Row(Seq())
+    )
+    checkAnswer(
+      df.select(regexp_extract_all($"s", "(a+)(b)?(c)", 2)),
+      Row(Seq(""))
+    )
   }
 
   test("string ascii function") {