Raise on empty targets instead of issuing a no-op request

Calling ``get_nearest_continuous`` with an empty ``targets`` is
almost always a caller bug (an unfiltered frame, a typo, a mis-named
column). The previous code papered over it by firing a trivial-range
HTTP request (``time=1900-01-01/1900-01-01``) purely so the caller
received a real ``BaseMetadata`` object. That pattern wastes a
round-trip on a nonsensical input and hides the bug.

Raise ``ValueError`` on empty ``targets`` instead. Shrinks the body
and makes a caller mistake loud.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: thodson-usgs

dataretrieval/waterdata/api.py

@@ -565,16 +565,7 @@ def get_nearest_continuous(
     window_td = pd.Timedelta(window)
 
     if len(targets) == 0:
-        # Issue a trivial-range request so the caller still receives a
-        # real ``BaseMetadata``; return an empty frame with the same
-        # shape a real response would have.
-        df, md = get_continuous(
-            monitoring_location_id=monitoring_location_id,
-            parameter_code=parameter_code,
-            time="1900-01-01T00:00:00Z/1900-01-01T00:00:00Z",
-            **kwargs,
-        )
-        return _empty_nearest_result(df), md
+        raise ValueError("targets must contain at least one timestamp")
 
     filter_expr = _build_window_or_filter(targets, window_td)
     df, md = get_continuous(

tests/waterdata_nearest_test.py

@@ -171,16 +171,13 @@ def test_multi_site_returns_row_per_target_per_site(patch_get_continuous):
     assert set(result["monitoring_location_id"]) == {"USGS-1", "USGS-2"}
 
 
-def test_empty_targets_returns_empty_frame_without_building_filter(
-    patch_get_continuous,
-):
-    patch_get_continuous.return_value = (_fake_df([]), mock.Mock())
-    result, _ = get_nearest_continuous([], monitoring_location_id="USGS-02238500")
-    assert result.empty
-    # The one call that happens uses a trivial time= window, not a filter.
-    _, kwargs = patch_get_continuous.call_args
-    assert "filter" not in kwargs
-    assert "time" in kwargs
+def test_empty_targets_raises(patch_get_continuous):
+    """An empty ``targets`` is a call with no useful work to do and
+    almost always a caller bug — raise rather than silently issuing a
+    no-op HTTP request."""
+    with pytest.raises(ValueError, match="targets"):
+        get_nearest_continuous([], monitoring_location_id="USGS-02238500")
+    patch_get_continuous.assert_not_called()
 
 
 def test_rejects_time_kwarg(patch_get_continuous):