MDEV-6052 Inconsistent results with bit type

Remove the "don't update the row for b'' and store uninitialized bytes on disk" change.
Update test cases to allow DEFAULT b'', because b'' is a valid expression elsewhere.

mysql-test/r/type_bit.result

@@ -722,7 +722,12 @@ DROP TABLE t1;
 CREATE TABLE IF NOT EXISTS t1 (
 f1 bit(2) NOT NULL default b''
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1 COLLATE=latin1_general_ci;
-ERROR 42000: Invalid default value for 'f1'
+SHOW CREATE TABLE t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `f1` bit(2) NOT NULL DEFAULT b'0'
+) ENGINE=MyISAM DEFAULT CHARSET=latin1 COLLATE=latin1_general_ci
+DROP TABLE t1;
 create table t1bit7 (a1 bit(7) not null) engine=MyISAM;
 create table t2bit7 (b1 bit(7)) engine=MyISAM;
 insert into t1bit7 values (b'1100000');

mysql-test/r/type_bit_innodb.result

@@ -413,3 +413,12 @@ a
 `
 drop table t1;
 End of 5.0 tests
+create table t1(f1 bit(2) not null default b'10',f2 bit(14) not null default b'11110000111100');
+insert into t1 (f1) values (default);
+insert into t1 values (b'',b''),('','');
+select hex(f1), hex(f2) from t1;
+hex(f1)	hex(f2)
+2	3C3C
+0	0
+0	0
+drop table t1;

mysql-test/t/type_bit.test

@@ -362,11 +362,11 @@ f2 bit(14) NOT NULL default b'11110000111100'
 SHOW CREATE TABLE t1;
 DROP TABLE t1;
 
---error ER_INVALID_DEFAULT
 CREATE TABLE IF NOT EXISTS t1 (
 f1 bit(2) NOT NULL default b''
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1 COLLATE=latin1_general_ci;
-
+SHOW CREATE TABLE t1;
+DROP TABLE t1;
 
 #
 # Bug#31399 Wrong query result when doing join buffering over BIT fields 

mysql-test/t/type_bit_innodb.test

@@ -148,3 +148,12 @@ select * from t1;
 drop table t1;
 
 --echo End of 5.0 tests
+
+#
+# MDEV-6052 Inconsistent results with bit type
+#
+create table t1(f1 bit(2) not null default b'10',f2 bit(14) not null default b'11110000111100');
+insert into t1 (f1) values (default);
+insert into t1 values (b'',b''),('','');
+select hex(f1), hex(f2) from t1;
+drop table t1;

sql/item.cc

@@ -6389,8 +6389,6 @@ int Item_hex_hybrid::save_in_field(Field *field, bool no_conversions)
 
   ulonglong nr;
   uint32 length= str_value.length();
-  if (!length)
-    return 1;
 
   if (length > 8)
   {