JSON and XML both support UTF-8. Neither supports embedding arbitrary binary data directly, especially if that data is not valid in your current character set
You wanted to send zero byte. This is how you send zero byte. You are making unrelated claims now that has nothing to do with your original claim.
You can't safely send zero bytes over XML even with UTF-8 encoding. Not in practice:
echo '<zero>�</zero>' | xmllint -
-:1: parser error : xmlParseCharRef: invalid xmlChar value 0
<zero>�</zero>
^
printf '<?xml version="1.0" encoding="utf-8"?><zero>\0</zero>' | xmllint -
-:1: parser error : Premature end of data in tag zero line 1
<?xml version="1.0" encoding="utf-8"?><zero>
^ Character Range
[2] Char ::= [#x1-#xD7FF] | [#xE000-#xFFFD] | [#x10000-#x10FFFF] /* any Unicode character, excluding the surrogate blocks, FFFE, and FFFF. */
[2a] RestrictedChar ::= [#x1-#x8] | [#xB-#xC] | [#xE-#x1F] | [#x7F-#x84] | [#x86-#x9F]
Your understanding of "easier" is oversimplified to the point that it's wrong. It's easier to do the wrong thing in JSON, it's harder to do the right thing in JSON (compared to XML).
JSON is a poorly thought-out format. It's problems become progressively more difficult to deal with the more you expect of your program.