Raise ValueError if concatenate is given no arrays (dask#4927)

* Raise `ValueError` if `concatenate` gets no arrays

NumPy will raise if no arrays are provided to concatenate as it is
unclear what to do. This adds a similar exception for Dask Arrays. Also
this short circuits handling unusual cases later. Plus raises a clearer
exception than one might see if this weren't raised.

* Test `concatenate` raises when no arrays are given

dask/array/core.py

@@ -2982,6 +2982,10 @@ def concatenate(seq, axis=0, allow_unknown_chunksizes=False):
     seq = [asarray(a) for a in seq]
 
     n = len(seq)
+
+    if n == 0:
+        raise ValueError("Need array(s) to concatenate")
+
     ndim = len(seq[0].shape)
 
     if axis < 0:

dask/array/tests/test_array_core.py

@@ -357,6 +357,7 @@ def test_concatenate():
     assert (concatenate([a, b, c], axis=-1).chunks ==
             concatenate([a, b, c], axis=1).chunks)
 
+    pytest.raises(ValueError, lambda: concatenate([]))
     pytest.raises(ValueError, lambda: concatenate([a, b, c], axis=2))