Add parentheses to fix UnexpectedLoaderException message logic

These were missing in 5211ff0dc1

Without these, the `": null"` is still logged (but the `"Unexpected
IllegalStateException"` bit is not), because the **whole** string
concatenation is compared to null as the boolean condition of the
ternary, and this condition is always false (the result of a string
concatentation is never `null`) i.e. (with excess parentheses for
clarity):

```
(("Unexpected " + cause.getClass().getSimpleName() + cause.getMessage()) != null)
    ? ": " + cause.getMessage()
    : ""
```

Also add a test because obviously this isn't as simple as I'd thought.

PiperOrigin-RevId: 593079975

libraries/exoplayer/src/main/java/androidx/media3/exoplayer/upstream/Loader.java

@@ -48,9 +48,9 @@ public final class Loader implements LoaderErrorThrower {
 
     public UnexpectedLoaderException(Throwable cause) {
       super(
-          "Unexpected " + cause.getClass().getSimpleName() + cause.getMessage() != null
-              ? ": " + cause.getMessage()
-              : "",
+          "Unexpected "
+              + cause.getClass().getSimpleName()
+              + (cause.getMessage() != null ? ": " + cause.getMessage() : ""),
           cause);
     }
   }

libraries/exoplayer/src/test/java/androidx/media3/exoplayer/upstream/UnexpectedLoaderExceptionTest.java

@@ -0,0 +1,48 @@
+/*
+ * Copyright 2023 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package androidx.media3.exoplayer.upstream;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import androidx.media3.exoplayer.upstream.Loader.UnexpectedLoaderException;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/** Tests for {@link UnexpectedLoaderException}. */
+@RunWith(JUnit4.class)
+public class UnexpectedLoaderExceptionTest {
+
+  @Test
+  public void causeWithMessage_messageAppended() {
+    UnexpectedLoaderException unexpectedLoaderException =
+        new UnexpectedLoaderException(new IllegalStateException("test message"));
+
+    assertThat(unexpectedLoaderException)
+        .hasMessageThat()
+        .isEqualTo("Unexpected IllegalStateException: test message");
+  }
+
+  @Test
+  public void causeWithoutMessage_noMessageAppended() {
+    UnexpectedLoaderException unexpectedLoaderException =
+        new UnexpectedLoaderException(new IllegalStateException());
+
+    assertThat(unexpectedLoaderException)
+        .hasMessageThat()
+        .isEqualTo("Unexpected IllegalStateException");
+  }
+}