Handle instruments: null at waypoints (#300)

* handle null for instruments

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* add test that empty list and null are equivalent and return no time

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

---------

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

Author: j-atkins

src/virtualship/utils.py

@@ -619,9 +619,9 @@ def _calc_wp_stationkeeping_time(
     """For a given waypoint (and the instruments present at this waypoint), calculate how much time is required to carry out all instrument deployments."""
     from virtualship.instruments.types import InstrumentType  # avoid circular imports
 
-    assert isinstance(wp_instrument_types, list), (
-        "waypoint instruments must be provided as a list, even if empty."
-    )
+    # to empty list if wp instruments set to 'null'
+    if not wp_instrument_types:
+        wp_instrument_types = []
 
     # TODO: this can be removed if/when CTD and CTD_BGC are merged to a single instrument
     both_ctd_and_bgc = (

tests/test_utils.py

@@ -347,3 +347,16 @@ class DrifterConfig:
     assert stationkeeping_time_xbt == datetime.timedelta(0), (
         "XBT should have zero stationkeeping time"
     )
+
+
+def test_calc_wp_stationkeeping_time_no_instruments(expedition):
+    """Test calc_wp_stationkeeping_time handles no instruments, either marked as 'null' or empty list."""
+    stationkeeping_emptylist = _calc_wp_stationkeeping_time(
+        [], expedition.instruments_config
+    )
+    stationkeeping_null = _calc_wp_stationkeeping_time(
+        None, expedition.instruments_config
+    )  # "null" in YAML translates to None in Python
+
+    assert stationkeeping_null == stationkeeping_emptylist  # are equivalent
+    assert stationkeeping_null == datetime.timedelta(0)  # at least one is 0 time