e.GetFilteredPolicy Empty string treated as wildcard

[DatScreamer]

Expected behaviour:
In GetFilteredPolicy, "" empty string should be treated as empty string, and "*" should be treated as wildcard.

Actual behaviour:
"" empty string acts as a wildcard in this line of GetFilteredPolicy:
if fieldValue != "" && rule[fieldIndex+i] != fieldValue {

Suggested fix:
GetStrictFilteredPolicy which treats "" empty string as empty string, and treats "*" as wildcard.

GetStrictFilteredPolicy so as to not introduce a breaking change in GetFilteredPolicy

Why does this matter:

1. It's been discussed that Casbin adapters should support empty strings, therefore there should be a way to Get Filtered Policy for rules with empty string values.
2. Semantic accuracy.