Allow empty string "" in columns to be overridden with default values in priors (#384)

The check only looked for whether it was not a string, but empty string values that are passed as `""` were not caught here. This should fix the error.
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Co-authored-by: Daniel Weindl <daniel.weindl@uni-bonn.de>

Author: PaulJonasJost

petab/v1/priors.py

@@ -277,9 +277,11 @@ def from_par_dict(
             at the bounds. **deprecated**.
         :return: A distribution object.
         """
-        dist_type = d.get(f"{type_}PriorType", C.PARAMETER_SCALE_UNIFORM)
-        if not isinstance(dist_type, str) and np.isnan(dist_type):
-            dist_type = C.PARAMETER_SCALE_UNIFORM
+        dist_type = C.PARAMETER_SCALE_UNIFORM
+        if (_table_dist_type := d.get(f"{type_}PriorType")) and (
+            isinstance(_table_dist_type, str) or not np.isnan(_table_dist_type)
+        ):
+            dist_type = _table_dist_type
 
         pscale = d.get(C.PARAMETER_SCALE, C.LIN)
         params = d.get(f"{type_}PriorParameters", None)