Android: Throw exception in CallSessionFileRotatingLogSink if dir is null

This CL throws an IllegalArgumentException in case the dir path argument
is null. This makes the error more clear than crashing in native JNI
code while trying to convert a null string.

Bug: b/106732994
Change-Id: Ib04ebf017c6e33b9896fc1e1db051a853838a7f4
Reviewed-on: https://webrtc-review.googlesource.com/81740
Reviewed-by: Paulina Hensman <phensman@webrtc.org>
Commit-Queue: Magnus Jedvert <magjed@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23535}

sdk/android/api/org/webrtc/CallSessionFileRotatingLogSink.java

@@ -14,11 +14,17 @@ public class CallSessionFileRotatingLogSink {
   private long nativeSink;
 
   public static byte[] getLogData(String dirPath) {
+    if (dirPath == null) {
+      throw new IllegalArgumentException("dirPath may not be null.");
+    }
     return nativeGetLogData(dirPath);
   }
 
   public CallSessionFileRotatingLogSink(
       String dirPath, int maxFileSize, Logging.Severity severity) {
+    if (dirPath == null) {
+      throw new IllegalArgumentException("dirPath may not be null.");
+    }
     nativeSink = nativeAddSink(dirPath, maxFileSize, severity.ordinal());
   }