Fix for bug #31349: ERROR 1062 (23000): Duplicate entry '' for key 'group_key'

Problem: lying to the optimizer that a function (Item_func_inet_ntoa)
cannot return NULL values leads to unexpected results (in the case group
keys creation/comparison is broken). 

Fix: Item_func_inet_ntoa::maybe_null should be set properly.

mysql-test/r/func_misc.result

@@ -185,4 +185,10 @@ ERROR 21000: Operand should contain 1 column(s)
 drop table table_26093;
 drop function func_26093_a;
 drop function func_26093_b;
+create table t1 (a int not null);
+insert into t1 values (-1), (-2);
+select min(a) from t1 group by inet_ntoa(a);
+min(a)
+-2
+drop table t1;
 End of 5.0 tests

mysql-test/t/func_misc.test

@@ -189,4 +189,12 @@ drop table table_26093;
 drop function func_26093_a;
 drop function func_26093_b;
 
+#
+# Bug #31349: ERROR 1062 (23000): Duplicate entry '' for key 'group_key'
+#
+create table t1 (a int not null);
+insert into t1 values (-1), (-2);
+select min(a) from t1 group by inet_ntoa(a);
+drop table t1;
+
 --echo End of 5.0 tests

sql/item_strfunc.h

@@ -683,7 +683,12 @@ class Item_func_inet_ntoa : public Item_str_func
     }
   String* val_str(String* str);
   const char *func_name() const { return "inet_ntoa"; }
-  void fix_length_and_dec() { decimals = 0; max_length=3*8+7; }
+  void fix_length_and_dec() 
+  { 
+    decimals= 0; 
+    max_length= 3 * 8 + 7; 
+    maybe_null= 1;
+  }
 };
 
 class Item_func_quote :public Item_str_func