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.

Some parameters, in modules, have a specific data type, but allow the
use of an empty string to clear the parameter.

By providing a method to retrieve the parameter with the correct data
type, or optionally an empty string, allows for consistency of parameter
handling between different modules.

Use the commom parameter type handling method for parameters that accept
a value or an empty string.

Use the commom parameter type handling method for parameters that accept
a value or an empty string.

Use the commom parameter type handling method for parameters that accept
a value or an empty string.