Remove encoding unlimited_dims if Time not present

When xarray 2025.08.0 opens a file with an empty unlimited
Time dimension but no variables with Time, it drops `Time` as a
dimension but keeps it in `unlimited_dims`.  This is probably
not an expected behavior but it's easy enough to address by
setting `unlimited_dims = None` if `Time` is not present.

Author: xylar

conda_package/mpas_tools/io.py

@@ -135,6 +135,8 @@ def write_netcdf(
         # make sure the Time dimension is unlimited because MPAS has trouble
         # reading Time otherwise
         ds.encoding['unlimited_dims'] = {'Time'}
+    else:
+        ds.encoding['unlimited_dims'] = None
 
     # for performance, we have to handle this as a special case
     convert = format == 'NETCDF3_64BIT_DATA'