Address a few nullness mismatches that our checker newly detects.

We had concluded way back in cl/63478563 that the `Platform` one is harmless in practice, and the `RegularImmutableMap` one is definitely harmless. Still, they're good to address.

RELNOTES=n/a
PiperOrigin-RevId: 625103048

android/guava/src/com/google/common/base/Platform.java

@@ -38,7 +38,17 @@ static CharMatcher precomputeCharMatcher(CharMatcher matcher) {
 
   static <T extends Enum<T>> Optional<T> getEnumIfPresent(Class<T> enumClass, String value) {
     WeakReference<? extends Enum<?>> ref = Enums.getEnumConstants(enumClass).get(value);
-    return ref == null ? Optional.<T>absent() : Optional.of(enumClass.cast(ref.get()));
+    /*
+     * We use `fromNullable` instead of `of` because `WeakReference.get()` has a nullable return
+     * type.
+     *
+     * In practice, we are very unlikely to see `null`: The `WeakReference` to the enum constant
+     * won't be cleared as long as the enum constant is referenced somewhere, and the enum constant
+     * is referenced somewhere for as long as the enum class is loaded. *Maybe in theory* the enum
+     * class could be unloaded after the above call to `getEnumConstants` but before we call
+     * `get()`, but that is vanishingly unlikely.
+     */
+    return ref == null ? Optional.absent() : Optional.fromNullable(enumClass.cast(ref.get()));
   }
 
   static String formatCompact4Digits(double value) {

android/guava/src/com/google/common/collect/RegularImmutableMap.java

@@ -406,7 +406,7 @@ public UnmodifiableIterator<Entry<K, V>> iterator() {
     }
 
     @Override
-    int copyIntoArray(Object[] dst, int offset) {
+    int copyIntoArray(@Nullable Object[] dst, int offset) {
       return asList().copyIntoArray(dst, offset);
     }
 
@@ -537,7 +537,7 @@ public UnmodifiableIterator<K> iterator() {
     }
 
     @Override
-    int copyIntoArray(Object[] dst, int offset) {
+    int copyIntoArray(@Nullable Object[] dst, int offset) {
       return asList().copyIntoArray(dst, offset);
     }
 

guava/src/com/google/common/base/Platform.java

@@ -42,7 +42,17 @@ static CharMatcher precomputeCharMatcher(CharMatcher matcher) {
 
   static <T extends Enum<T>> Optional<T> getEnumIfPresent(Class<T> enumClass, String value) {
     WeakReference<? extends Enum<?>> ref = Enums.getEnumConstants(enumClass).get(value);
-    return ref == null ? Optional.<T>absent() : Optional.of(enumClass.cast(ref.get()));
+    /*
+     * We use `fromNullable` instead of `of` because `WeakReference.get()` has a nullable return
+     * type.
+     *
+     * In practice, we are very unlikely to see `null`: The `WeakReference` to the enum constant
+     * won't be cleared as long as the enum constant is referenced somewhere, and the enum constant
+     * is referenced somewhere for as long as the enum class is loaded. *Maybe in theory* the enum
+     * class could be unloaded after the above call to `getEnumConstants` but before we call
+     * `get()`, but that is vanishingly unlikely.
+     */
+    return ref == null ? Optional.absent() : Optional.fromNullable(enumClass.cast(ref.get()));
   }
 
   static String formatCompact4Digits(double value) {