Anim: Remove empty FCurves from layered Actions

This changes the behavior when deleting the last key of an FCurve on layered actions.
Previously the FCurve would continue to exist, whereas now it is deleted.
This makes it consistent with legacy actions.

I modified the "Clear Keyframes" operator in this PR as well to make it work with layered actions.

Pull Request: https://projects.blender.org/blender/blender/pulls/125327

tests/python/bl_animation_keyframing.py

@@ -688,6 +688,42 @@ class NlaInsertTest(AbstractKeyframingTest, unittest.TestCase):
         self.assertAlmostEqual(fcurve_loc_x.keyframe_points[-1].co[1], 1.0, 8)
 
 
+class KeyframeDeleteTest(AbstractKeyframingTest, unittest.TestCase):
+
+    def test_delete_in_v3d_pose_mode(self):
+        armature = _create_armature()
+        bpy.context.scene.frame_set(1)
+        with bpy.context.temp_override(**_get_view3d_context()):
+            bpy.ops.anim.keyframe_insert_by_name(type="Location")
+        self.assertTrue(armature.animation_data is not None)
+        self.assertTrue(armature.animation_data.action is not None)
+        action = armature.animation_data.action
+        self.assertEqual(len(action.fcurves), 3)
+
+        bpy.ops.object.mode_set(mode='POSE')
+        with bpy.context.temp_override(**_get_view3d_context()):
+            bpy.ops.anim.keyframe_insert_by_name(type="Location")
+            bpy.context.scene.frame_set(5)
+            bpy.ops.anim.keyframe_insert_by_name(type="Location")
+            # This should have added new FCurves for the pose bone.
+            self.assertEqual(len(action.fcurves), 6)
+
+            bpy.ops.anim.keyframe_delete_v3d()
+            # No Fcurves should yet be deleted.
+            self.assertEqual(len(action.fcurves), 6)
+            self.assertEqual(len(action.fcurves[0].keyframe_points), 1)
+            bpy.context.scene.frame_set(1)
+            bpy.ops.anim.keyframe_delete_v3d()
+            # This should leave the object level keyframes of the armature
+            self.assertEqual(len(action.fcurves), 3)
+
+        bpy.ops.object.mode_set(mode='OBJECT')
+        with bpy.context.temp_override(**_get_view3d_context()):
+            bpy.ops.anim.keyframe_delete_v3d()
+        # The last FCurves should be deleted from the object now.
+        self.assertEqual(len(action.fcurves), 0)
+
+
 def main():
     global args
     import argparse