Return early if there are no junctions to threshold.

Previously, if there were no junctions after filtering and k_threshold was None, you'd get a `ValueError` when trying to max over an empty array. Instead, return early if there are no junctions after filtering.

PiperOrigin-RevId: 875753172
Change-Id: I679ffad56a87ef1864826b9f7c4d1c7129a97015

Author: tomwardio

src/alphagenome/data/junction_data.py

@@ -244,10 +244,14 @@ def get_junctions_to_plot(
     raise ValueError(
         f'Expected only one ontology term, got {filtered.num_tracks}.'
     )
+  filtered_junctions = []
+  if filtered.values.size == 0:
+    return filtered_junctions
+
   if k_threshold is None:
     k_threshold = filtered.values.max() * 0.05
+
   # Round k for better visualization.
-  filtered_junctions = []
   for interval, k in zip(filtered.junctions, filtered.values, strict=True):
     # Filter by threshold and strand.
     if interval.strand != strand:

src/alphagenome/data/junction_data_test.py

@@ -223,6 +223,12 @@ def test_intersect_with_interval(self):
               genome.Junction('chr2', 10, 11, '+', k=5),
           ],
       ),
+      dict(
+          name='unknown_track_name',
+          k_threshold=None,
+          strand='+',
+          expected=[],
+      ),
   ])
   def test_get_junctions_to_plot(self, name, k_threshold, strand, expected):
     metadata = pd.DataFrame({