Support for default values

[jan888adams]

This merge request introduces the option to specify a default value for a property in case the source value is null. The key changes include:

Configuration.php:

Users can now use the keyword 'source' to define the source property and 'default' to specify a default value. If no default value is needed, the old syntax can still be used.

NeustaConverterExtension.php:

Modified NeustaConverterExtension.php to pass the default value property to the DefaultMappingPopulator by reading the default property.

PropertyMappingPopulator.php:

In PropertyMappingPopulator.php, a new constructor parameter of type 'mixed' has been added, allowing any type (string, bool, int, null, etc.) to be passed as the default value. When the source property returns null, it will be replaced with the provided default value.

Example Configuration:

neusta_converter:
  converter:
    person.converter:
      target_factory: AppBundle\Factory\PersonFactory
      properties:
        firstname:
          default: John
        lastname: 
          source: name
          default: Doe
        email: ~
        phone: phoneNumber

Resolves Default value (PropertyMappingPopulator) #49 issue.

[mike4git]

I'm fine with - especially the new syntax