Fix NullPointerException when java home not provided (#5151)

Fixes #5149

Someone I can't get a reliably repro of that failure, but this PR should
theoretically fix the mechanism through which the failure happens by
making the launcher-daemon protocol robust against null strings

---------

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

Author: lihaoyi

runner/client/src/mill/client/ClientUtil.java

@@ -65,6 +65,7 @@ public static Map<String, String> parseMap(InputStream argStream) throws IOExcep
   public static String readString(InputStream inputStream) throws IOException {
     // Result is between 0 and 255, hence the loop.
     final int length = readInt(inputStream);
+    if (length == -1) return null;
     final byte[] arr = new byte[length];
     int total = 0;
     while (total < length) {
@@ -78,9 +79,13 @@ public static String readString(InputStream inputStream) throws IOException {
   }
 
   public static void writeString(OutputStream outputStream, String string) throws IOException {
-    final byte[] bytes = string.getBytes(utf8);
-    writeInt(outputStream, bytes.length);
-    outputStream.write(bytes);
+    if (string == null) {
+      writeInt(outputStream, -1);
+    } else {
+      final byte[] bytes = string.getBytes(utf8);
+      writeInt(outputStream, bytes.length);
+      outputStream.write(bytes);
+    }
   }
 
   public static void writeInt(OutputStream out, int i) throws IOException {

runner/client/test/src/mill/client/ClientTests.java

@@ -48,7 +48,12 @@ public void readWriteInt() throws Exception {
   @Test
   public void readWriteString() throws Exception {
     String[] examples = {
-      "", "hello", "i am cow", "i am cow\nhear me moo\ni weight twice as much as you", "我是一个叉烧包",
+      "",
+      "hello",
+      "i am cow",
+      "i am cow\nhear me moo\ni weight twice as much as you",
+      "我是一个叉烧包",
+      null
     };
     for (String example : examples) {
       checkStringRoundTrip(example);