wagl metadata: Fall back to nbart time range when nbar not available

Now that nbar is being disabled for Landat scenes, this change is
required so that we still include the time range in the metadata.

Author: jeremyh

eodatasets3/wagl.py

@@ -458,15 +458,22 @@ def _create_contiguity(
     p: DatasetAssembler,
     product_list: Iterable[str],
     resolution_yx: Tuple[float, float],
-    timedelta_product: str = "nbar",
-    timedelta_data: numpy.ndarray = None,
+    timedelta_products: Iterable[str] = ("nbar", "nbart"),
+    timedelta_data: Optional[numpy.ndarray] = None,
 ):
     """
     Create the contiguity (all pixels valid) dataset.
 
     Write a contiguity mask file based on the intersection of valid data pixels across all
     bands from the input files.
     """
+
+    # Calculate the time range from the first that we have available
+    timedelta_product = next(
+        (preferred for preferred in timedelta_products if preferred in product_list),
+        None,
+    )
+
     for product in product_list:
         contiguity = None
         # A contiguity layer for each product