Refine offense range for Lint/SafeNavigationWithEmpty

This PR refines offense range for `Lint/SafeNavigationWithEmpty`.

The following is an example.

```ruby
return unless foo&.empty?
```

## Before

```console
% bundle exec rubocop example.rb --only Lint/SafeNavigationWithEmpty
(snip)

Offenses:

example.rb:1:1: W: Lint/SafeNavigationWithEmpty: Avoid calling empty?
with the safe navigation operator in conditionals.
return unless foo&.empty?
^^^^^^^^^^^^^^^^^^^^^^^^^

1 file inspected, 1 offense detected
```

## After

```console
% bundle exec rubocop example.rb --only Lint/SafeNavigationWithEmpty
(snip)

Offenses:

example.rb:1:15: W: Lint/SafeNavigationWithEmpty: Avoid calling empty?
with the safe navigation operator in conditionals.
return unless foo&.empty?
              ^^^^^^^^^^^

1 file inspected, 1 offense detected
```

lib/rubocop/cop/lint/safe_navigation_with_empty.rb

@@ -30,7 +30,7 @@ class SafeNavigationWithEmpty < Cop
         def on_if(node)
           return unless safe_navigation_empty_in_conditional?(node)
 
-          add_offense(node)
+          add_offense(node.condition)
         end
       end
     end

spec/rubocop/cop/lint/safe_navigation_with_empty_spec.rb

@@ -7,7 +7,7 @@
     it 'registers an offense on `&.empty?`' do
       expect_offense(<<~RUBY)
         return unless foo&.empty?
-        ^^^^^^^^^^^^^^^^^^^^^^^^^ Avoid calling `empty?` with the safe navigation operator in conditionals.
+                      ^^^^^^^^^^^ Avoid calling `empty?` with the safe navigation operator in conditionals.
       RUBY
     end