[Fix rubocop#10711] Fix an error for Style/MultilineTernaryOperator

lib/rubocop/cop/style/multiline_ternary_operator.rb

@@ -7,7 +7,7 @@ module Style
       #
       # NOTE: `return if ... else ... end` is syntax error. If `return` is used before
       # multiline ternary operator expression, it will be autocorrected to single-line
-      # ternary operator. The same is true for `break`, `next`, and method call.
+      # ternary operator. The same is true for `break`, `next`, method call, and without assignment.
       #
       # @example
       #   # bad
@@ -73,7 +73,8 @@ def replacement(node)
         end
 
         def enforce_single_line_ternary_operator?(node)
-          SINGLE_LINE_TYPES.include?(node.parent.type) && !use_assignment_method?(node.parent)
+          node.parent.nil? ||
+            (SINGLE_LINE_TYPES.include?(node.parent.type) && !use_assignment_method?(node.parent))
         end
 
         def use_assignment_method?(node)

spec/rubocop/cop/style/multiline_ternary_operator_spec.rb

@@ -108,6 +108,18 @@
     RUBY
   end
 
+  it 'register an offense and corrects when returning a multiline ternary operator expression without assignment' do
+    expect_offense(<<~RUBY)
+      cond ? foo :
+      ^^^^^^^^^^^^ Avoid multi-line ternary operators, use single-line instead.
+             bar
+    RUBY
+
+    expect_correction(<<~RUBY)
+      cond ? foo : bar
+    RUBY
+  end
+
   it 'register an offense and corrects when returning a multiline ternary operator expression with `return`' do
     expect_offense(<<~RUBY)
       return cond ?