Utils: reject empty input in get_yields

[jayaddison]

Resolves #1313.

[jknndy]

When I apply these changes to #1307 test case _3 is still expecting 0 servings . Not sure what's causing it yet

[jayaddison]

Ah, I think it's that this str(...) typecast turns a None value into the string 'None'.

That input -- and a bunch of other nonsense inputs, seem to return 0 servings:

>>> from recipe_scrapers._utils import get_yields
>>> get_yields('None')
'0 servings'
>>> get_yields('N one')
'0 servings'
>>> get_yields('N')
'0 servings'
>>> get_yields('n')
'0 servings'
>>> get_yields('adsf')
'0 servings'

[jayaddison]

I think we can prevent the typecasting of None to 'None' -- but I can't think of an easy way to validate other non-empty strings.