Default null content type to application/json (#1223)

diff --git a/lottie/src/main/java/com/airbnb/lottie/network/NetworkFetcher.java b/lottie/src/main/java/com/airbnb/lottie/network/NetworkFetcher.java
index e0aa373..c3d31c7 100644
--- a/lottie/src/main/java/com/airbnb/lottie/network/NetworkFetcher.java
+++ b/lottie/src/main/java/com/airbnb/lottie/network/NetworkFetcher.java
@@ -135,7 +135,13 @@
     File file;
     FileExtension extension;
     LottieResult<LottieComposition> result = null;
-    switch (connection.getContentType()) {
+    String contentType = connection.getContentType();
+    if (contentType == null) {
+      // Assume JSON for best effort parsing. If it fails, it will just deliver the parse exception
+      // in the result which is more useful than failing here.
+      contentType = "application/json";
+    }
+    switch (contentType) {
       case "application/zip":
         Logger.debug("Handling zip response.");
         extension = FileExtension.ZIP;