cleanup: Field_set::empty_set_string

in particular, it overwrites pre-allocated buffer in val_buffer,
so following val_buffer->append()'s cause totally unnecessary
mallocs.

sql/field.cc

@@ -9381,16 +9381,6 @@ String *Field_set::val_str(String *val_buffer,
   ulonglong tmp=(ulonglong) Field_enum::val_int();
   uint bitnr=0;
 
-  /*
-    Some callers expect *val_buffer to contain the result,
-    so we assign to it, rather than doing 'return &empty_set_string.
-  */
-  *val_buffer= empty_set_string;
-  if (tmp == 0)
-  {
-    return val_buffer;
-  }
-
   val_buffer->set_charset(field_charset());
   val_buffer->length(0);
 

sql/field.h

@@ -4809,8 +4809,7 @@ class Field_set final :public Field_enum {
             const LEX_CSTRING *field_name_arg, uint32 packlength_arg,
 	    const TYPELIB *typelib_arg, const DTCollation &collation)
     :Field_enum(ptr_arg, len_arg, null_ptr_arg, null_bit_arg, unireg_check_arg,
-                field_name_arg, packlength_arg, typelib_arg, collation),
-      empty_set_string("", 0, collation.collation)
+                field_name_arg, packlength_arg, typelib_arg, collation)
     {
       flags=(flags & ~ENUM_FLAG) | SET_FLAG;
     }
@@ -4833,8 +4832,6 @@ class Field_set final :public Field_enum {
   { return &type_handler_set; }
   bool has_charset() const override { return true; }
   Binlog_type_info binlog_type_info() const override;
-private:
-  const String empty_set_string;
 };