Fix default 'alt' value in bp_core_fetch_avatar_filter

[jefferai]

In bp_core_fetch_avatar_filter, the function information says that alt defaults to blank if nothing is specified. Nothing actually sets this, however, leading to errors in some cases. This trivial patch simply actually does default assignment if nothing is passed in.

[boonebgorges]

(In [4387]) Adds default value for param in bp_core_fetch_avatar_filter. Fixes #3225. Props jefferai