Update raw_metadata.py

Changed isinstance to is.
Fixed condition triggering warning.

nd2reader/raw_metadata.py

@@ -164,11 +164,11 @@ class RawMetadata(object):
         z_levels = self._parse_dimension(r""".*?Z\((\d+)\).*?""")
         if 0 == len(z_levels):
             z_levels = parse_if_not_none(self.z_data, self._parse_z_coordinates)
-            if isinstance(z_levels, type(None)):
+            if z_levels is None:
                 z_levels = []
             else:
                 z_levels = range(len(z_levels))
-            if 0 != z_levels:
+            if 0 != len(z_levels):
                 warnings.warn("Z-levels details missing in metadata. Using Z-coordinates instead.")
         return z_levels