Fix UnitStrippedWarning for non arrays (#1909)

* check magnitude is array
* numpy check

pint/facets/numpy/quantity.py

@@ -16,7 +16,7 @@
 from ..plain import PlainQuantity, MagnitudeT
 
 from ..._typing import Shape
-from ...compat import _to_magnitude, np
+from ...compat import _to_magnitude, np, HAS_NUMPY
 from ...errors import DimensionalityError, PintTypeError, UnitStrippedWarning
 from .numpy_func import (
     HANDLED_UFUNCS,
@@ -115,11 +115,12 @@ def _numpy_method_wrap(self, func, *args, **kwargs):
         return value
 
     def __array__(self, t=None) -> np.ndarray:
-        warnings.warn(
-            "The unit of the quantity is stripped when downcasting to ndarray.",
-            UnitStrippedWarning,
-            stacklevel=2,
-        )
+        if HAS_NUMPY and isinstance(self._magnitude, np.ndarray):
+            warnings.warn(
+                "The unit of the quantity is stripped when downcasting to ndarray.",
+                UnitStrippedWarning,
+                stacklevel=2,
+            )
         return _to_magnitude(self._magnitude, force_ndarray=True)
 
     def clip(self, min=None, max=None, out=None, **kwargs):