fix: fix ignore_missing_instruments option not working properly

The `get_note_end_time` function would try to get the note's sound data to estimate its length, but it would fail in case the sound file was missing, since it doesn't get added to the sounds dict returned by `load_instruments` at all (unlike when it's not assigned, when it's just set to `None`).

Fixes #3

nbswave/main.py

@@ -115,14 +115,17 @@ def get_length(self, notes: Sequence[nbs.Note]) -> float:
         """
 
         def get_note_end_time(note: nbs.Note) -> float:
-            sound = self._instruments[note.instrument]
-            note_pitch = audio.key_to_pitch(note.key)
 
             note_start = note.tick / self._song.header.tempo * 1000
-            note_length = len(sound) / note_pitch
-            note_end = note_start + note_length
-
-            return note_end
+            sound = self._instruments.get(note.instrument)
+
+            if sound is None:  # Sound either missing or not assigned
+                return note_start
+            else:
+                note_pitch = audio.key_to_pitch(note.key)
+                note_length = len(sound) / note_pitch
+                note_end = note_start + note_length
+                return note_end
 
         return max(get_note_end_time(note) for note in notes)