Remove confusing null from ParserException.getMessage()

When reading quickly this suggests something 'real' was null (similar to
a `NullPointerException`), but it's actually just the message from the
superclass.

Seen in stack trace in Issue: #2074:

```
Caused by: androidx.media3.common.ParserException: null {contentIsMalformed=true, dataType=1}
```

PiperOrigin-RevId: 719240235

libraries/common/src/main/java/androidx/media3/common/ParserException.java

@@ -109,11 +109,11 @@ protected ParserException(
     this.dataType = dataType;
   }
 
-  @Nullable
   @Override
   public String getMessage() {
-    return super.getMessage()
-        + " {contentIsMalformed="
+    String superMessage = super.getMessage();
+    return (superMessage != null ? superMessage + " " : "")
+        + "{contentIsMalformed="
         + contentIsMalformed
         + ", dataType="
         + dataType