from_binary produces a confusing error when slc_files is empty

[gmarupilla]

If slc_files is an empty iterable, from_binary silently skips the read loop, slcs stays None, and the eventual ds.assign({vlabel: (("azimuth","range","time"), None)}) surfaces as:

ValueError: Variable 'complex': Could not convert tuple of form
(dims, data[, attrs, encoding]): (('azimuth', 'range', 'time'), None) to Variable.

It took me a while to trace this back to "no files were found". This is easy to hit when the user runs the tutorial notebook before downloading the dataset — path.rglob('*.raw') returns [] and the failure mode is opaque.

An early guard would make the failure obvious:

if len(slc_files) == 0:
    raise ValueError("`slc_files` is empty; no SLC files to read.")

(Filed in the context of JOSS review openjournals/joss-reviews#10492.)

[gmarupilla]

Cross-ref: this is one of the failure modes triggered by #89 (the tutorial notebook expects a hand-downloaded dataset, and an empty file list lands here).