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

Fixes rubocop#10711.

This PR fixes an error for `Style/MultilineTernaryOperator`
when the false branch is on a separate line.

changelog/fix_an_error_for_style_multiline_ternary_operator.md

@@ -0,0 +1 @@
+* [#10711](https://github.com/rubocop/rubocop/issues/10711): Fix an error for `Style/MultilineTernaryOperator` when the false branch is on a separate line. ([@koic][])

lib/rubocop/cop/style/multiline_ternary_operator.rb

@@ -73,7 +73,7 @@ def replacement(node)
         end
 
         def enforce_single_line_ternary_operator?(node)
-          SINGLE_LINE_TYPES.include?(node.parent.type) && !use_assignment_method?(node.parent)
+          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

@@ -18,7 +18,7 @@
     RUBY
   end
 
-  it 'registers an offense and corrects when the false branch is on a separate line' do
+  it 'registers an offense and corrects when the false branch is on a separate line and assigning a return value' do
     expect_offense(<<~RUBY)
       a = cond ? b :
           ^^^^^^^^^^ Avoid multi-line ternary operators, use `if` or `unless` instead.
@@ -34,6 +34,22 @@
     RUBY
   end
 
+  it 'registers an offense and corrects when the false branch is on a separate line' do
+    expect_offense(<<~RUBY)
+      cond ? b :
+      ^^^^^^^^^^ Avoid multi-line ternary operators, use `if` or `unless` instead.
+      c
+    RUBY
+
+    expect_correction(<<~RUBY)
+      if cond
+        b
+      else
+        c
+      end
+    RUBY
+  end
+
   it 'registers an offense and corrects when everything is on a separate line' do
     expect_offense(<<~RUBY)
       a = cond ?