Fix null-constrait errors in mealie import

[Chronophylos]

Fix following error during mealie import:

ERROR 
null value in column "servings" of relation "cookbook_recipe" violates not-null constraint
DETAIL:  Failing row contains (304, Bechamel-Sauce, , , null, 0, t, 2024-10-12 13:47:31.576329+00, 2024-10-12 13:47:31.599914+00, 1, null, , 0, null, null, null, für 0,5 Liter Soße, None, 1, '0':2C '5':3C 'fur':1C 'liter':4C 'soss':5C, 'bechamel':2A 'bechamel-sauc':1A 'sauc':3A, null, t, f).

The error is caused by "recipe_yield": null in the exported recipes.

[CLAassistant]

All committers have signed the CLA.

[vabene1111]

thank you, I should probably fix the servings parse function to return 0 if nothing is found but this fixes it as well for now.