fix: handle nil member in MembershipFeeHelpers

- get_last_completed_cycle/2 and get_current_cycle/2 return nil when member is nil.
- Avoids FunctionClauseError when MemberLive.Show receives no member (e.g. after
  redirect or policy filter). Add unit tests for nil member.

test/mv_web/helpers/membership_fee_helpers_test.exs

@@ -68,6 +68,11 @@ defmodule MvWeb.Helpers.MembershipFeeHelpersTest do
   end
 
   describe "get_last_completed_cycle/2" do
+    test "returns nil when member is nil" do
+      assert MembershipFeeHelpers.get_last_completed_cycle(nil) == nil
+      assert MembershipFeeHelpers.get_last_completed_cycle(nil, Date.utc_today()) == nil
+    end
+
     test "returns last completed cycle for member", %{actor: actor} do
       # Create test data
       fee_type =
@@ -184,6 +189,11 @@ defmodule MvWeb.Helpers.MembershipFeeHelpersTest do
   end
 
   describe "get_current_cycle/2" do
+    test "returns nil when member is nil" do
+      assert MembershipFeeHelpers.get_current_cycle(nil) == nil
+      assert MembershipFeeHelpers.get_current_cycle(nil, Date.utc_today()) == nil
+    end
+
     test "returns current cycle for member", %{actor: actor} do
       fee_type =
         Mv.MembershipFees.MembershipFeeType