Avoid passing null configs to Bitmap.createBitmap in LruBitmapPool.

We already handle null configs for our various strategies because some 
types of Bitmaps (GIFs mostly) on some API levels will have special 
native configs that have a null Java equivalent. As a result, 
LruBitmapPool does not throw as long as there is a matching Bitmap in 
the pool even if the config we’re asked for is null. 

This change fixes the inconsistent behavior where we would throw if
the pool happened to not have a matching Bitmap when the given config
is null. Now we universally handle null configs in get() and getDirty().

library/src/main/java/com/bumptech/glide/load/engine/bitmap_recycle/LruBitmapPool.java

@@ -128,7 +128,7 @@ public Bitmap get(int width, int height, Bitmap.Config config) {
       // contents individually, so we do so here. See issue #131.
       result.eraseColor(Color.TRANSPARENT);
     } else {
-      result = Bitmap.createBitmap(width, height, config);
+      result = createBitmap(width, height, config);
     }
 
     return result;
@@ -139,11 +139,16 @@ public Bitmap get(int width, int height, Bitmap.Config config) {
   public Bitmap getDirty(int width, int height, Bitmap.Config config) {
     Bitmap result = getDirtyOrNull(width, height, config);
     if (result == null) {
-      result = Bitmap.createBitmap(width, height, config);
+      result = createBitmap(width, height, config);
     }
     return result;
   }
 
+  @NonNull
+  private static Bitmap createBitmap(int width, int height, @Nullable Bitmap.Config config) {
+    return Bitmap.createBitmap(width, height, config != null ? config : DEFAULT_CONFIG);
+  }
+
   @TargetApi(Build.VERSION_CODES.O)
   private static void assertNotHardwareConfig(Bitmap.Config config) {
     // Avoid short circuiting on sdk int since it breaks on some versions of Android.
@@ -159,7 +164,8 @@ private static void assertNotHardwareConfig(Bitmap.Config config) {
   }
 
   @Nullable
-  private synchronized Bitmap getDirtyOrNull(int width, int height, Bitmap.Config config) {
+  private synchronized Bitmap getDirtyOrNull(
+      int width, int height, @Nullable Bitmap.Config config) {
     assertNotHardwareConfig(config);
     // Config will be null for non public config types, which can lead to transformations naively
     // passing in null as the requested config here. See issue #194.

library/src/test/java/com/bumptech/glide/load/engine/bitmap_recycle/LruBitmapPoolTest.java

@@ -138,6 +138,18 @@ public void testPassesArgb8888ToStrategyAsConfigForRequestsWithNullConfigsOnGetD
     assertEquals(expected, result);
   }
 
+  @Test
+  public void get_withNullConfig_andEmptyPool_returnsNewArgb8888Bitmap() {
+    Bitmap result = pool.get(100, 100, /*config=*/ null);
+    assertThat(result.getConfig()).isEqualTo(Bitmap.Config.ARGB_8888);
+  }
+
+  @Test
+  public void getDirty_withNullConfig_andEmptyPool_returnsNewArgb8888Bitmap() {
+    Bitmap result = pool.getDirty(100, 100, /*config=*/ null);
+    assertThat(result.getConfig()).isEqualTo(Bitmap.Config.ARGB_8888);
+  }
+
   private void testTrimMemory(int fillSize, int trimLevel, int expectedSize) {
     MockStrategy strategy = new MockStrategy();
     LruBitmapPool pool = new LruBitmapPool(MAX_SIZE, strategy, ALLOWED_CONFIGS);
@@ -240,7 +252,7 @@ public void put(Bitmap bitmap) {
 
     @Override
     public Bitmap get(int width, int height, Bitmap.Config config) {
-      return bitmaps.removeLast();
+      return bitmaps.isEmpty() ? null : bitmaps.removeLast();
     }
 
     @Override