Do not crash when there are no active LSM modules

Reading ".target.string" could return nil instead of "" when read path
does not exist.

src/lib/y2security/lsm/config.rb

@@ -126,7 +126,7 @@ def read
       def active
         return [] unless Yast::Stage.normal
 
-        modules = Yast::SCR.Read(Yast.path(".target.string"), RUNNING_PATH)
+        modules = Yast::SCR.Read(Yast.path(".target.string"), RUNNING_PATH).to_s
         modules.split(",").each_with_object([]) do |name, result|
           supported_module = supported.find { |m| m.id.to_s == name }
           result << supported_module if supported_module

test/y2security/lsm/config_test.rb

@@ -66,6 +66,14 @@
       expect(active.first.id).to eql(:selinux)
     end
 
+    context "when there are no active LSM modules" do
+      let(:active_modules) { nil }
+
+      it "returns an empty array" do
+        expect(subject.active).to eq([])
+      end
+    end
+
     context "when no supported LSM is active" do
       let(:active_modules) { "lockdown,capabilities,tomoyo" }