fix(explore): Guard against missing attributes in validation response

Add nullish coalescing fallback when accessing the attributes field
from the validation API response. The prior optional chaining on
data[0]?.attributes could pass undefined to Object.entries() if
the response body lacked an attributes field, causing a TypeError.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: nsdeschenes

static/app/views/explore/hooks/useAttributeValidation.tsx

@@ -117,7 +117,7 @@ export function useAttributeValidation(
       return EMPTY_KEYS;
     }
 
-    return Object.entries(data?.[0]?.attributes)
+    return Object.entries(data[0]?.attributes ?? {})
       .filter(([, result]) => !result.valid)
       .map(([key]) => key);
   }, [data, hasValidation]);