Skip null check when creating main scheduler

This is a tradeoff to allow replacing the main scheduler in tests without needing Robolectric.

Author: JakeWharton

rxandroid/build.gradle

@@ -12,6 +12,10 @@ android {
         sourceCompatibility rootProject.ext.sourceCompatibility
         targetCompatibility rootProject.ext.sourceCompatibility
     }
+
+    testOptions {
+        unitTests.returnDefaultValues = true
+    }
 }
 
 dependencies {

rxandroid/src/main/java/io/reactivex/rxjava3/android/schedulers/AndroidSchedulers.java

@@ -25,8 +25,7 @@
 public final class AndroidSchedulers {
 
     private static final class MainHolder {
-        static final Scheduler DEFAULT
-            = from(Looper.getMainLooper());
+        static final Scheduler DEFAULT = internalFrom(Looper.getMainLooper(), true);
     }
 
     private static final Scheduler MAIN_THREAD =
@@ -61,10 +60,13 @@ public static Scheduler from(Looper looper) {
      *              locking. On API < 16 this value is ignored.
      * @see Message#setAsynchronous(boolean)
      */
-    @SuppressLint("NewApi") // Checking for an @hide API.
     public static Scheduler from(Looper looper, boolean async) {
         if (looper == null) throw new NullPointerException("looper == null");
+        return internalFrom(looper, async);
+    }
 
+    @SuppressLint("NewApi") // Checking for an @hide API.
+    private static Scheduler internalFrom(Looper looper, boolean async) {
         // Below code exists in androidx-core as well, but is left here rather than include an
         // entire extra dependency.
         // https://developer.android.com/reference/kotlin/androidx/core/os/MessageCompat?hl=en#setAsynchronous(android.os.Message,%20kotlin.Boolean)