gpu: Return early when attempting to bind 0 samplers.

This is a common occurrence for vertex shaders specifically, and allows backends
to not have to handle this edge case every time.

src/gpu/SDL_gpu.c

@@ -2109,6 +2109,10 @@ void SDL_BindGPUVertexSamplers(
         }
     }
 
+    if (num_bindings == 0) {
+        return;
+    }
+
     RENDERPASS_DEVICE->BindVertexSamplers(
         RENDERPASS_COMMAND_BUFFER,
         first_slot,
@@ -2216,6 +2220,10 @@ void SDL_BindGPUFragmentSamplers(
         }
     }
 
+    if (num_bindings == 0) {
+        return;
+    }
+
     RENDERPASS_DEVICE->BindFragmentSamplers(
         RENDERPASS_COMMAND_BUFFER,
         first_slot,