Fall back to "loading" placeholder when all else fails.

Fixes regression from #5306

Similar to how regular [Glide requests
work](https://github.com/bumptech/glide/blob/c9cc739ce84b204defb1933e5f5f47d7c4a530b7/library/src/main/java/com/bumptech/glide/request/BaseRequestOptions.java#L304-L305),
the "loading" placeholder is expected to be used if there's a null model
and null "error" placeholder.

GlideImage.kt used to mention this: "The resource and `Drawable`
variants will be displayed if the request fails and no other failure
handling is specified".  Presumably, this verbiage was removed because
Composeable placeholders were being broken, and for no other reason.

Author: donaldchai

integration/compose/src/androidTest/java/com/bumptech/glide/integration/compose/GlideImageErrorTest.kt

@@ -50,6 +50,23 @@ class GlideImageErrorTest {
       .assert(expectDisplayedDrawable(errorDrawable))
   }
 
+  @Test
+  fun loadingParameter_withError_isUsedWhenAllElseFails() {
+    val description = "test"
+    val resourceId = android.R.drawable.star_big_off
+    glideComposeRule.setContent {
+      GlideImage(
+        model = null,
+        contentDescription = description,
+        loading = placeholder(resourceId),
+      )
+    }
+
+    glideComposeRule
+      .onNodeWithContentDescription(description)
+      .assert(expectDisplayedPainter(context, resourceId))
+  }
+
   @Test
   fun failureParameter_withErrorResourceId_displaysError() {
     val description = "test"
@@ -58,6 +75,7 @@ class GlideImageErrorTest {
       GlideImage(
         model = null,
         contentDescription = description,
+        loading = placeholder(android.R.drawable.star_on), // arbitary; shouldn't be displayed.
         failure = placeholder(failureResourceId),
       )
     }

integration/compose/src/main/java/com/bumptech/glide/integration/compose/GlideModifier.kt

@@ -434,7 +434,7 @@ internal class GlideNode : DrawModifierNode, LayoutModifierNode, SemanticsModifi
                 // Prefer the override Painters if set.
                 val painter = when (state) {
                   is RequestState.Loading -> loadingPlaceholder
-                  is RequestState.Failure -> errorPlaceholder
+                  is RequestState.Failure -> errorPlaceholder ?: loadingPlaceholder
                   is RequestState.Success -> throw IllegalStateException()
                 }
                 val primary = if (painter != null) {