glTF exporter: fix animation export when not sampled and constraints

io_scene_gltf2/__init__.py

@@ -15,7 +15,7 @@
 bl_info = {
     'name': 'glTF 2.0 format',
     'author': 'Julien Duroure, Norbert Nopper, Urs Hanselmann, Moritz Becher, Benjamin Schmithüsen, Jim Eckerlein, and many external contributors',
-    "version": (0, 9, 14),
+    "version": (0, 9, 15),
     'blender': (2, 80, 0),
     'location': 'File > Import-Export',
     'description': 'Import-Export as glTF 2.0',

io_scene_gltf2/blender/exp/gltf2_blender_gather_animation_samplers.py

@@ -99,6 +99,10 @@ def __gather_non_keyed_values(channels: typing.Tuple[bpy.types.FCurve],
         "value": 1
     }.get(target)
 
+    if length is None:
+        # This is not a known target
+        return ()
+
     for i in range(0, length):
         if bake_channel is not None:
             non_keyed_values.append({