Guard hitlet entropy test from numerical errors (#772)

* Guard hitlet entropy test from numerical errors

Avoid waveforms for which normalization is not reliable in float32

* Restore NaN assert for zero waveforms

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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---------

Co-authored-by: Dacheng Xu <dx2227@columbia.edu>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

tests/test_hitlet.py

@@ -464,7 +464,16 @@ def test_conditional_entropy(data, size_template_and_ind_max_template):
 
     # Test 1.: Flat template and no data:
     e1 = strax.conditional_entropy(hitlet, "flat")[0]
-    if np.sum(data):
+
+    sum_data = np.sum(data)
+    if sum_data:
+        if np.abs(sum_data) < 1e-4 * np.ptp(data):
+            # Normalizing may cause significant float32 numerical errors.
+            # Do not run any tests: this data is unsuitable for testing since
+            # slight numpy <-> numba implementation differences could cause
+            # failures that are actually harmless.
+            return
+
         d = data
         d = d / np.sum(d)
         m = d > 0