test: Add test for embedded null in hex string

Also, fix style in the corresponding function. The style change can be
reviewed with "--word-diff-regex=."

src/util/strencodings.cpp

@@ -78,7 +78,6 @@ bool IsHexNumber(std::string_view str)
 
 std::vector<unsigned char> ParseHex(std::string_view str)
 {
-    // convert hex dump to vector
     std::vector<unsigned char> vch;
     auto it = str.begin();
     while (it != str.end() && it + 1 != str.end()) {