CopyFromStagingToBuffer: ASSERT size is not 0

Zero-sized copies are invalid in a couple backends, and in follow up
CLs CreateBufferMapped will be change to handle zero-sized buffers
correctly.

Bug: chromium:1069076
Change-Id: Ieef62a13182bbe1e939a3847980c91339e42aa8f
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/22460
Reviewed-by: Stephen White <senorblanco@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

src/dawn_native/Buffer.cpp

diff --git a/src/dawn_native/Buffer.cpp b/src/dawn_native/Buffer.cpp
index 2b738b2..46d00d1 100644
--- a/src/dawn_native/Buffer.cpp
+++ b/src/dawn_native/Buffer.cpp
@@ -319,6 +319,10 @@
 
     MaybeError BufferBase::CopyFromStagingBuffer() {
         ASSERT(mStagingBuffer);
+        if (GetSize() == 0) {
+            return {};
+        }
+
         DAWN_TRY(GetDevice()->CopyFromStagingToBuffer(mStagingBuffer.get(), 0, this, 0, GetSize()));
 
         DynamicUploader* uploader = GetDevice()->GetDynamicUploader();