[JsonWriter] Do not break in EscapeString when encountering a null by…

…te (grpc#33127)

Instead just Utf-16 encode the null byte when dumping the value to a
string form.

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src/core/lib/json/json_writer.cc

@@ -145,9 +145,7 @@ void JsonWriter::EscapeString(const std::string& string) {
   OutputChar('"');
   for (size_t idx = 0; idx < string.size(); ++idx) {
     uint8_t c = static_cast<uint8_t>(string[idx]);
-    if (c == 0) {
-      break;
-    } else if (c >= 32 && c <= 126) {
+    if (c >= 32 && c <= 126) {
       if (c == '\\' || c == '"') OutputChar('\\');
       OutputChar(static_cast<char>(c));
     } else if (c < 32 || c == 127) {

test/core/json/corpus/testcase-6037245441474560

@@ -0,0 +1 @@
+"\u0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"

test/core/json/corpus/testcase-6329722588758016

@@ -0,0 +1 @@
+"\u0000"