Clean up DRM post requests

- Explicitly specify HTTP_METHOD_POST (previously this was
  implicit as a result of the body data being non-null)
- Use null when there's no body data (it's converted to
  null inside of the DataSpec constructor anyway)

PiperOrigin-RevId: 256573384

library/core/src/main/java/com/google/android/exoplayer2/drm/HttpMediaDrmCallback.java

@@ -111,7 +111,7 @@ public final class HttpMediaDrmCallback implements MediaDrmCallback {
   public byte[] executeProvisionRequest(UUID uuid, ProvisionRequest request) throws IOException {
     String url =
         request.getDefaultUrl() + "&signedRequest=" + Util.fromUtf8Bytes(request.getData());
-    return executePost(dataSourceFactory, url, Util.EMPTY_BYTE_ARRAY, null);
+    return executePost(dataSourceFactory, url, /* httpBody= */ null, /* requestProperties= */ null);
   }
 
   @Override
@@ -139,7 +139,7 @@ public final class HttpMediaDrmCallback implements MediaDrmCallback {
   private static byte[] executePost(
       HttpDataSource.Factory dataSourceFactory,
       String url,
-      byte[] data,
+      @Nullable byte[] httpBody,
       @Nullable Map<String, String> requestProperties)
       throws IOException {
     HttpDataSource dataSource = dataSourceFactory.createDataSource();
@@ -154,7 +154,8 @@ public final class HttpMediaDrmCallback implements MediaDrmCallback {
       DataSpec dataSpec =
           new DataSpec(
               Uri.parse(url),
-              data,
+              DataSpec.HTTP_METHOD_POST,
+              httpBody,
               /* absoluteStreamPosition= */ 0,
               /* position= */ 0,
               /* length= */ C.LENGTH_UNSET,