Check isEmpty() instead of size == 0.

That way is simpler.

It's also somewhat better for users who pass a filtered collection, **but:**
- We may undo this change in the future (as we are considering approaches that would actually use the result of `size` for more than an emptiness check).
- A nonempty filtered collection will still apply the filter at least once during the `isEmpty` call, so that can still cause trouble if the filter has side effects or "call the filter until it returns `true`" is slow enough to matter.
- Other methods in Guava likely still trigger similar behavior when given a filtered collection, as do methods outside of Guava.

Fixes #7143 (for now)

RELNOTES=n/a
PiperOrigin-RevId: 623232817

guava/src/com/google/common/collect/ImmutableSet.java

@@ -189,8 +189,7 @@ public static <E> ImmutableSet<E> copyOf(Collection<? extends E> elements) {
       return copyOfEnumSet((EnumSet<?>) elements);
     }
 
-    int size = elements.size();
-    if (size == 0) {
+    if (elements.isEmpty()) {
       // We avoid allocating anything.
       return of();
     }