FIX: Don't warn on empty .discourse-compatibility (#16032)

lib/version.rb

@@ -31,7 +31,7 @@ module Discourse
   #  2.4.4.beta6: some-other-branch-ref
   #  2.4.2.beta1: v1-tag
   def self.find_compatible_resource(version_list, version = ::Discourse::VERSION::STRING)
-    return unless version_list
+    return unless version_list.present?
 
     begin
       version_list = YAML.safe_load(version_list)

spec/lib/version_spec.rb

@@ -67,6 +67,10 @@ describe Discourse::VERSION do
       expect(Discourse.find_compatible_resource(nil)).to be_nil
     end
 
+    it "returns nil when empty" do
+      expect(Discourse.find_compatible_resource("")).to be_nil
+    end
+
     it "raises an error on invalid input" do
       expect { Discourse.find_compatible_resource("1.0.0.beta1 12f82d5") }.to raise_error(Discourse::InvalidVersionListError)
     end