Add a guard for MTLDevice.counterSets being null.

This is a tentative fix for a segfault that's happening on
metal::Adapter initialization on some systems.

Bug: dawn:1102
Change-Id: Iff31e46420e60cd76ecbbfa92bd5b5906ca89cf4
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/69340
Auto-Submit: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Austin Eng <enga@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>

src/dawn_native/metal/BackendMTL.mm

@@ -194,12 +194,14 @@ namespace dawn_native { namespace metal {
             // MTLDevice’s counterSets property declares which counter sets it supports. Check
             // whether it's available on the device before requesting a counter set.
             id<MTLCounterSet> counterSet = nil;
+            if (device.counterSets != nil) {
                 for (id<MTLCounterSet> set in device.counterSets) {
                     if ([set.name caseInsensitiveCompare:counterSetName] == NSOrderedSame) {
                         counterSet = set;
                         break;
                     }
                 }
+            }
 
             // The counter set is not supported.
             if (counterSet == nil) {