More descriptive variable name
Don't convert zero values to empty string

PHP's empty() function returns true for 0, 0.00, "0.00" and other 0 variants.
That means woocommerce_process_shop_order_meta() will convert item meta with
a 0.00 or similar value to an empty string, which can break things that expect
a float or number. This commit fixes that.
Merge pull request #2262 from thenbrent/shop_order_meta

Don't convert zero values to empty string in woocommerce_process_shop_order_meta()
rejig the hooks Closes #2263.
Closes #2261.
Fix multiselect name Closes #2259
Fix saving as array #2259