Update gsp.py (#384)

* Update gsp.py

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

src/gsp.py

@@ -90,6 +90,8 @@ def get_all_available_forecasts(
 
     if isinstance(gsp_ids, str):
         gsp_ids = [int(gsp_id) for gsp_id in gsp_ids.split(",")]
+        if gsp_ids == "":
+            gsp_ids = None
 
     if is_fake():
         if gsp_ids is None: