Always transpile nullish coalescing operator because it cannot be processed by webpack 4

[niklasnatter]

What's in this PR?

This PR adds the @babel/plugin-proposal-nullish-coalescing-operator plugin to our babel configuration to transpile the code of the react-leaflet package. Without this change, the build errors with the following message when installing the newest version of the react-leaflet package:

ERROR in /sulu-skeleton/vendor/sulu/sulu/node_modules/react-leaflet/esm/Pane.js 25:37
Module parse failed: Unexpected token (25:37)
You may need an appropriate loader to handle this file type, currently no loaders are configured to process this file. See https://webpack.js.org/concepts#loaders
|   }
| 
>   const parentPaneName = props.pane ?? context.pane;
|   const parentPane = parentPaneName ? context.map.getPane(parentPaneName) : undefined;
|   const element = context.map.createPane(name, parentPane);
 @ /sulu-skeleton/vendor/sulu/sulu/node_modules/react-leaflet/esm/index.js 13:0-30 13:0-30
 @ /sulu-skeleton/vendor/sulu/sulu/src/Sulu/Bundle/LocationBundle/Resources/js/containers/Location/Location.js
 @ /sulu-skeleton/vendor/sulu/sulu/src/Sulu/Bundle/LocationBundle/Resources/js/containers/Location/index.js
 @ /sulu-skeleton/vendor/sulu/sulu/src/Sulu/Bundle/LocationBundle/Resources/js/containers/Form/fields/Location.js
 @ /sulu-skeleton/vendor/sulu/sulu/src/Sulu/Bundle/LocationBundle/Resources/js/containers/Form/index.js
 @ /sulu-skeleton/vendor/sulu/sulu/src/Sulu/Bundle/LocationBundle/Resources/js/index.js
 @ ./index.js

Related Issues

• Add @babel/plugin-proposal-nullish-coalescing-operator to transpile code of react-leaflet package sulu#6056
• Nullish coalescing operator makes package incompatible with webpack 4 PaulLeCam/react-leaflet#883