Improve get_acq_constraint sentinel value with a proper never-matching regex

Agent-Logs-Url: https://github.com/khanlab/SPIMprep/sessions/91f24ab5-f565-4a53-8e6f-33d6d4b26771

Co-authored-by: akhanf <11492701+akhanf@users.noreply.github.com>

Author: Copilot

workflow/rules/common.smk

@@ -271,7 +271,9 @@ def get_acq_constraint(input_type_str):
     explicit input_type column rather than fragile substring matching on acq."""
     acqs = samples[samples["input_type"] == input_type_str]["acq"].unique().tolist()
     if not acqs:
-        return "NOMATCH_EMPTY"
+        # Return a regex that matches nothing when no samples use this input_type;
+        # this prevents the rule from matching any wildcard value.
+        return "(?!x)x"
     return "|".join(re.escape(acq) for acq in acqs)