[Export] REGEXP restriction fails on empty list

[CBroz1]

A late addition to the export process was the inclusion of externals tables using a REGEXP restriction

spyglass/src/spyglass/common/common_usage.py

Lines 222 to 224 in 239f2a5

	analysis_restr = ( # filepaths have analysis subdir. regexp substrings
	"filepath REGEXP '" + "|".join(self._list_analysis_files(key)) + "'"
	) # regexp is slow, but we're only doing this once, and future-proof

If there are no such files, the empty regex will cause an error

[CBroz1]

Fix for future PR

class ExportSelection(SpyglassMixin, dj.Manual):
    ...

    def _add_externals_to_restr_graph(
        self, restr_graph: RestrGraph, key: dict
    ) -> RestrGraph:
        """Add external tables to a RestrGraph for a given restriction/key.

        Tables added as nodes with restrictions based on file paths. Names
        added to visited set to appear in restr_ft obj bassed to SQLDumpHelper.

        Parameters
        ----------
        restr_graph : RestrGraph
            A RestrGraph object to add external tables to.
        key : dict
            Any valid restriction key for ExportSelection.Table

        Returns
        -------
        restr_graph : RestrGraph
            The updated RestrGraph
        """
        if raw_files := self._list_raw_files(key):
            raw_tbl = self._externals["raw"]
            raw_name = raw_tbl.full_table_name
            raw_restr = "filepath in ('" + "','".join(raw_files) + "')"
            restr_graph.graph.add_node(raw_name, ft=raw_tbl, restr=raw_restr)
            restr_graph.visited.add(raw_name)

        if analysis_files := self._list_analysis_files(key):
            analysis_tbl = self._externals["analysis"]
            analysis_name = analysis_tbl.full_table_name
            # to avoid issues with analysis subdir, we use REGEXP
            # this is slow, but we're only doing this once, and future-proof
            analysis_restr = "filepath REGEXP '" + "|".join(analysis_files) + "'"
            restr_graph.graph.add_node(
                analysis_name, ft=analysis_tbl, restr=analysis_restr
            )
            restr_graph.visited.add(analysis_name)

        return restr_graph