Return empty diagnostics result if there are no syntax errors and rubocop isn't installed

lib/ruby_lsp/requests/diagnostics.rb

@@ -44,7 +44,7 @@ def initialize(document)
       def response
         # Running RuboCop is slow, so to avoid excessive runs we only do so if the file is syntactically valid
         return syntax_error_diagnostics if @document.syntax_error?
-        return unless defined?(Support::RuboCopDiagnosticsRunner)
+        return [] unless defined?(Support::RuboCopDiagnosticsRunner)
 
         Support::RuboCopDiagnosticsRunner.instance.run(@uri, @document).map!(&:to_lsp_diagnostic)
       end

test/requests/diagnostics_test.rb

@@ -26,4 +26,36 @@ def foo
     assert_equal(2, diagnostics.length)
     assert_equal("expected an `end` to close the `def` statement", T.must(diagnostics.last).message)
   end
+
+  def test_empty_diagnostics_without_rubocop
+    document = RubyLsp::RubyDocument.new(source: <<~RUBY, version: 1, uri: URI("file:///fake/file.rb"))
+      def foo
+        "Hello, world!"
+      end
+    RUBY
+
+    # We want to unload the rubocop runner for this test; first make sure that it's loaded
+    require "ruby_lsp/requests/support/rubocop_diagnostics_runner"
+    klass = RubyLsp::Requests::Support::RuboCopDiagnosticsRunner
+    RubyLsp::Requests::Support.send(:remove_const, :RuboCopDiagnosticsRunner)
+
+    diagnostics = T.must(RubyLsp::Requests::Diagnostics.new(document).response)
+
+    assert_empty(diagnostics)
+  ensure
+    # Restore the class
+    RubyLsp::Requests::Support.const_set(:RuboCopDiagnosticsRunner, klass)
+  end
+
+  def test_empty_diagnostics_with_rubocop
+    document = RubyLsp::RubyDocument.new(source: <<~RUBY, version: 1, uri: URI("file:///fake/file.rb"))
+      def foo
+        "Hello, world!"
+      end
+    RUBY
+
+    diagnostics = T.must(RubyLsp::Requests::Diagnostics.new(document).response)
+
+    refute_empty(diagnostics)
+  end
 end