Don't allow nullptr in texels array params (unless using a transfer buffer).

src/gpu/vk/GrVkGpu.cpp

Index: src/gpu/vk/GrVkGpu.cpp
diff --git a/src/gpu/vk/GrVkGpu.cpp b/src/gpu/vk/GrVkGpu.cpp
index 1ce78fd192a27d5899b6f8ab671f64f533005fc0..d357f32e1fe5dfe047cd8d41b1c819fe32003bd8 100644
--- a/src/gpu/vk/GrVkGpu.cpp
+++ b/src/gpu/vk/GrVkGpu.cpp
@@ -542,7 +542,8 @@ GrTexture* GrVkGpu::onCreateTexture(const GrSurfaceDesc& desc, GrGpuResource::Li
     }
 
     // TODO: We're ignoring MIP levels here.
-    if (!texels.empty() && texels.begin()->fPixels) {
+    if (!texels.empty()) {
+        SkASSERT(texels.begin()->fPixels);
         if (!this->uploadTexData(tex, 0, 0, desc.fWidth, desc.fHeight, desc.fConfig,
                                  texels.begin()->fPixels, texels.begin()->fRowBytes)) {
             tex->unref();