pw_stream_uart_mcuxpresso: Make dma_stream Write of size 0 succeed

This changes the behavior of the dma_stream implementation to always
have a write of size 0 succeed, rather than returning "invalid
argument". A write of size 0 is valid, and is required to be supported
by the RPC system. This also matches the behavior of the non-DMA stream.

Tested: Do a write of size 0, see that it succeeds
Change-Id: I8c5712ad130a69bf6fbe9645dd01ce04408bdfd8
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/214151
Reviewed-by: Austin Foxley <[email protected]>
Commit-Queue: Eli Lipsitz <[email protected]>
Lint: Lint 🤖 <[email protected]>
Reviewed-by: Christoph Klee <[email protected]>

pw_stream_uart_mcuxpresso/dma_stream.cc

@@ -478,7 +478,7 @@ StatusWithSize UartDmaStreamMcuxpresso::DoRead(ByteSpan data) {
 // the USART TX channel.
 Status UartDmaStreamMcuxpresso::DoWrite(ConstByteSpan data) {
   if (data.size() == 0) {
-    return Status::InvalidArgument();
+    return OkStatus();
   }
 
   bool was_busy = tx_data_.busy.exchange(true);