Update decode_time_units() to not run if time dimension does not exist

[tomvothecoder]

Is your feature request related to a problem? If yes, Please describe

Some datasets may not have a time dimension, so decode_time_units() should not run.

Describe your proposed solution

if "time" in ds.dims:
    ds = decode_time_units(ds)

Describe alternatives solutions you've considered

N/A

Additional context

N/A