Rename parameter 'allow_empty_string' to 'allow_empty_list_item'

The parameter 'allow_empty_string' in 'module_params_get' is used to
allow an item in a list to be an empty string. The problem is that the
naming is misleading, as it is checking a list item rather than a
string.

This patch rename the parameter to 'allow_empty_list_item' so that it
more clearly refers to list itens instead of standalone strings, and do
not collide with future parameters that may test for empty strings which
are not part of lists.