fix: issue a warning when validate_collections is called without collection (stac-utils#761)

Co-authored-by: Pete Gadomski <pete.gadomski@gmail.com>

Author: vprivat-ads

src/stac_api_validator/validations.py

@@ -1074,6 +1074,10 @@ def validate_collections(
                         r_session.headers,
                     )
 
+            if not collection:
+                warnings += f"[{Context.COLLECTIONS}] Collection parameter required for running all Collections validation"
+                return
+
             collection_url = f"{data_link['href']}/{collection}"
             _, body, resp_headers = retrieve(
                 Method.GET,