Use an array as a default value for $meta in wpmu_* functions

[SergeyBiryukov]

Background: #20793, ticket:24210:11, ticket:22187:12

There are a bunch of Multisite signup functions with $meta = '' as the default value, where it should really be an array (and is documented as such).

[24126] and [24262] fixed this for wp-signup.php, we should do the same in wp-includes/ms-functions.php for consistency.

[SergeyBiryukov]

In 25205:

Use an array as a default value for $meta in multisite signup-related functions. fixes #25149.