fix: allow no forecast variable

Author: gadomski

src/stactools/met_office_deterministic/href.py

@@ -119,7 +119,7 @@ def duration(self) -> str | None:
             return None
 
     @property
-    def variable(self) -> str:
+    def variable(self) -> str | None:
         """The CF-standard name for this parameter."""
         match self.parameter:
             case "cloud_amount_of_total_cloud":
@@ -274,7 +274,7 @@ def variable(self) -> str:
             case "landsea_mask":
                 return "land_binary_mask"
             case _:
-                raise ValueError(f"Unknown parameter: {self.parameter}")
+                return None
 
     def __str__(self) -> str:
         """Returns the href as a string.

src/stactools/met_office_deterministic/stac.py

@@ -152,9 +152,9 @@ def _create_item(
 
 
 def _create_asset(href: Href) -> tuple[str, Asset]:
-    extra_fields = {
-        "forecast:variable": href.variable,
-    }
+    extra_fields = {}
+    if variable := href.variable:
+        extra_fields["forecast:variable"] = variable
     if href.duration:
         extra_fields["forecast:duration"] = href.duration
     item_assets = _get_item_assets(href.model, href.theme)