MDEV-36565 Assertion `src != ((void *)0)' failed in my_casedn_8bit

The crash happened when ExtractValue() returning an empty string
as a result was passed to LCASE() or UCASE().

Item_func_xml_extractvalue::val_str() could return a String
{Ptr=0,str_value=0} in some cases, to mean an empty retult.
But virtual my_charset_handler_st functions caseup() and casedn()
do not expect {src=nullptr,srclen=0} as input and:
- raise a DBUG_ASSERT() in debug builds, or
- raise a "applying zero offset to null pointer" warning in UBSAN builds

Fixing Item_func_xml_extractvalue::val_str() to return
a String {Ptr="",str_length=0} instead of {Ptr=0,str_value=0}.

A similar fix was done earlier in Field_set::val_str(). See c69fb1a627.

mysql-test/main/xml.result

@@ -1320,5 +1320,17 @@ f
 foo
 <b></b>
 #
-# Start of 10.5 tests
+# End of 10.5 tests
 #
+# Start of 11.4 tests
+#
+# MDEV-36565 Assertion `src != ((void *)0)' failed in my_casedn_8bit
+#
+SET NAMES latin1;
+SELECT  lcase((extractvalue('a', 'a'))) a FROM (select 1) dt;
+a
+
+SELECT  ucase((extractvalue('a', 'a'))) a FROM (select 1) dt;
+a
+
+# End of 11.4 tests