Masked arrays break wind_direction function

[rpmanser]

Masked arrays break the wind_direction function when all calculated directions are greater than 0 degrees.

import numpy as np
import metpy.calc as mpcalc
from metpy.units import units

mask = np.array([True, False, True, False])

u = np.array([4., 2., 0., 0.])
v = np.array([0., 2., 4., 0.])

u_masked = units.Quantity(np.ma.array(u, mask=mask), units('m/s'))
v_masked = units.Quantity(np.ma.array(v, mask=mask), units('m/s'))

dirs = mpcalc.wind_direction(u_masked, v_masked)

Which results in the following error:
ValueError: can only convert an array of size 1 to a Python scalar

at this line of the function:

wdir[wdir <= 0] += 360. * units.deg

which only occurs when all values in wdir are greater than 0.

The traceback goes through the pint and numpy masked array libraries, so this may be an upstream problem. If different units are specified and a similar operation is performed, things work as expected. For example,

temperature = units.Quantity(np.ma.array([250., 260., 270., 280.], mask=mask), units.kelvin)
temperature[temperature <= 0.] += 10. * units.kelvin

works just fine.

A temporary work around could involve just checking if the magnitude of wdir is a masked array.

Linux Mint 18.3 Sylvia
MetPy version 1.0.0
NumPy version 1.18.4

[dopplershift]

I'm not sure why the other one works. If a boolean operation like that returns all False, using that to get items from an array returns an empty scalar, which we should not then be adding to. This should probably change to something like:

mask = wdir <= 0
if np.any(mask):
    wdir[mask] += ...

Wanna submit a PR to fix this?