Make [EnumMap(Map)](https://docs.oracle.com/en/java/javase/21/docs/…

…api/java.base/java/util/EnumMap.html#%3Cinit%3E(java.util.Map)) throw if passed an empty `Map` (other than an `EnumMap`).

My motivation is that I'm migrating Guava's tests to run under J2CL, and we have [a test](https://github.com/google/guava/blob/b567b26e2bd6ea679b8cf69711cc8a41ecf219c4/guava-tests/test/com/google/common/collect/MapsTest.java#L359-L366) that expects an `IllegalArgumentException` in this case. Now, that's really more a test of the implementation of `EnumMap` than it is of a test of Guava, so I could see deleting the test or at least not running it under J2CL. But I figured I'd at least run this CL by you first to see if you thought the behavior was worth considering changing. (Not that I love the JDK's design of this API, and not that J2CL actually needs a non-empty map.)

PiperOrigin-RevId: 570769285

jre/java/java/util/EnumMap.java

@@ -15,6 +15,7 @@
  */
 package java.util;
 
+import static javaemul.internal.InternalPreconditions.checkArgument;
 
 /**
  * A {@link java.util.Map} of {@link Enum}s. <a
@@ -33,6 +34,7 @@ public EnumMap(Class<K> type) {}
 
   public EnumMap(Map<K, ? extends V> m) {
     putAll(m);
+    checkArgument(m instanceof EnumMap || !isEmpty());
   }
 
   @Override

jre/javatests/com/google/j2cl/jre/java/util/EnumMapTest.java

@@ -208,4 +208,16 @@ public void testOrdering() {
       lastOrdinal = newOrdinal;
     }
   }
+
+  public void testConstructorSucceedsGivenEmptyEnumMap() {
+    var unused = new EnumMap<>(new EnumMap<Numbers, Integer>(Numbers.class));
+  }
+
+  public void testConstructorThrowsGivenEmptyOtherMap() {
+    try {
+      new EnumMap<>(new HashMap<Numbers, Integer>());
+      fail();
+    } catch (IllegalArgumentException expected) {
+    }
+  }
 }