unwrap_pyoptcol is missing except keyword that causes exceptions …

…ignored, fixes function bug (#719)

Fixes a bug in the cdef function when `pyoptcol` is `None` and gets mishandled (raised as error). This error is captured in one of the pytests for `point_linestring_distance`. However, due to the cdef function is missing the `except` keyword, the exception gets ignored by the python interpreter and not reraised.

Authors:
  - Michael Wang (https://github.com/isVoid)

Approvers:
  - H. Thomson Comer (https://github.com/thomcom)

URL: #719

python/cuspatial/cuspatial/_lib/utils.pyx

@@ -5,13 +5,15 @@ from cudf._lib.cpp.column.column_view cimport column_view
 from cuspatial._lib.cpp.optional cimport nullopt, optional
 
 
-cdef optional[column_view] unwrap_pyoptcol(pyoptcol):
+cdef optional[column_view] unwrap_pyoptcol(pyoptcol) except *:
     # Unwrap python optional Column arg to c optional[column_view]
     cdef Column pycol
-    cdef optional[column_view] c_opt = nullopt
+    cdef optional[column_view] c_opt
     if isinstance(pyoptcol, Column):
         pycol = pyoptcol
         c_opt = pycol.view()
+    elif pyoptcol is None:
+        c_opt = nullopt
     else:
-        raise ValueError("pyoptcol must be a Column or None")
+        raise ValueError("pyoptcol must be either Column or None.")
     return c_opt