Reject null in CharStreams.asWriter(appendable).write(string[, ...]).

This brings its behavior in line with other `Writer` implementations. Unfortunately, the docs don't describe this behavior:

https://docs.oracle.com/en/java/javase/14/docs/api/java.base/java/io/Writer.html#write(java.lang.String)

Note also that this behavior differs from the behavior of `Writer.append(null)`, which is to write the four characters "null." That behavior is itself poorly documented, since the docs for `append(CharSequence)` claim that it is equivalent to `out.write(csq.toString())`, which would throw NPE... though at least *that* is contradicted in the `@param` tag. (The difference between `write` and `append` presumably arises from inheriting `append` from the newer `Appendable` API.)

RELNOTES=`io`: Changed `CharStreams.asWriter(appendable).write(string[, ...])` to reject a null `string`.
PiperOrigin-RevId: 370516874

Author: cpovirk

android/guava/src/com/google/common/io/AppendableWriter.java

@@ -68,13 +68,15 @@ public void write(int c) throws IOException {
   }
 
   @Override
-  public void write(@NullableDecl String str) throws IOException {
+  public void write(String str) throws IOException {
+    checkNotNull(str);
     checkNotClosed();
     target.append(str);
   }
 
   @Override
-  public void write(@NullableDecl String str, int off, int len) throws IOException {
+  public void write(String str, int off, int len) throws IOException {
+    checkNotNull(str);
     checkNotClosed();
     // tricky: append takes start, end pair...
     target.append(str, off, off + len);

guava/src/com/google/common/io/AppendableWriter.java

@@ -68,13 +68,15 @@ public void write(int c) throws IOException {
   }
 
   @Override
-  public void write(@Nullable String str) throws IOException {
+  public void write(String str) throws IOException {
+    checkNotNull(str);
     checkNotClosed();
     target.append(str);
   }
 
   @Override
-  public void write(@Nullable String str, int off, int len) throws IOException {
+  public void write(String str, int off, int len) throws IOException {
+    checkNotNull(str);
     checkNotClosed();
     // tricky: append takes start, end pair...
     target.append(str, off, off + len);