BUG: convolve: fix incorrect error message for empty input - #30306

In [1]: np.convolve(a=[], v=[1])
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Cell In[1], line 1
----> 1 np.convolve(a=[], v=[1])

File ~/.local/share/uv/tools/ipython/lib/python3.14/site-packages/numpy/_core/numeric.py:902, in convolve(a, v, mode)
    900     raise ValueError('a cannot be empty')
    901 if len(v) == 0:
--> 902     raise ValueError('v cannot be empty')
    903 return multiarray.correlate(a, v[::-1], mode)

ValueError: v cannot be empty

In [1]: np.convolve(a=[], v=[1])
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Cell In[1], line 1
----> 1 np.convolve(a=[], v=[1])

File ~/Workspace/numpy/build-install/usr/lib/python3.11/site-packages/numpy/_core/numeric.py:897, in convolve(a, v, mode)
    895 a, v = array(a, copy=None, ndmin=1), array(v, copy=None, ndmin=1)
    896 if len(a) == 0:
--> 897     raise ValueError('a cannot be empty')
    898 if len(v) == 0:
    899     raise ValueError('v cannot be empty')

ValueError: a cannot be empty