Mark Rails/CompactBlank as unsafe

Follow up to #598.

This PR marks `Rails/CompactBlank` as unsafe.

It is unsafe by default because false positives may occur in the
blank check of block arguments to the receiver object.

```ruby
[[1, 2], [3, nil]].reject { |first, second| second.blank? }
# => [[1, 2]]

[[1, 2], [3, nil]].compact_blank
# => [[1, 2], [3, nil]]
```

Author: koic

config/default.yml

@@ -183,6 +183,7 @@ Rails/BulkChangeTable:
 Rails/CompactBlank:
   Description: 'Checks if collection can be blank-compacted with `compact_blank`.'
   Enabled: pending
+  Safe: false
   VersionAdded: '<<next>>'
 
 Rails/ContentTag:

lib/rubocop/cop/rails/compact_blank.rb

@@ -5,6 +5,14 @@ module Cop
     module Rails
       # Checks if collection can be blank-compacted with `compact_blank`.
       #
+      # @safety
+      #   It is unsafe by default because false positives may occur in the
+      #   blank check of block arguments to the receiver object.
+      #
+      #   For example, `[[1, 2], [3, nil]].reject { |first, second| second.blank? }` and
+      #   `[[1, 2], [3, nil]].compact_blank` are not compatible. The same is true for `empty?`.
+      #   This will work fine when the receiver is a hash object.
+      #
       # @example
       #
       #   # bad