Regex preprocessing of field fails on null value

[jrmhaig]

If a field is set as follows:

amoeba do
  enable
  regex test_field: { replace: /one/, with: 'two' }
end

then amoeba_dup fails if the value in the field is nil.

To reproduce

Create a migration:

class CreateTestModels < ActiveRecord::Migration[6.1]
  def change
    create_table :test_models do |t|
      t.string :test_string

      t.timestamps
    end
  end
end

and the corresponding model:

class TestModel < ApplicationRecord
  amoeba do
    enable
    regex test_field: { replace: /one/, with: 'two' }
  end
end

Then:

irb> test = TestModel.create
irb> test.test_string
=> nil
irb> dup = test.amoeba_dup

Expected behaviour: dup.test_string is set to nil, the same as test.test_string.
Actual behaviour: NoMethodError (undefined method gsub! for nil:NilClass) exception is raised at lib/amoeba/cloner.rb line 159.