mergeVersions doesn't work with 'next' flag

[ghost]

1. have a workspace where lineup is in two repos: one should have 'next', the other should have f.e. '4.1.1' as version
2. run 'yo phovea:update'
3. you will get the following error:
   TypeError: Cannot read property 'version' of null
   at /home/circleci/phovea/node_modules/generator-phovea/utils/NpmUtils.js:80:144
   at Array.map ()
   at Function.findMaxVersion (/home/circleci/phovea/node_modules/generator-phovea/utils/NpmUtils.js:80:90)
   at Function.mergeVersions (/home/circleci/phovea/node_modules/generator-phovea/utils/NpmUtils.js:38:34)
   at /home/circleci/phovea/node_modules/generator-phovea/generators/workspace/index.js:242:36
   at Array.forEach ()
   at Generator._generateWebDependencies (/home/circleci/phovea/node_modules/generator-phovea/generators/workspace/index.js:241:31)
   at Generator.writing (/home/circleci/phovea/node_modules/generator-phovea/generators/workspace/index.js:538:85)
   at Object. (/home/circleci/phovea/node_modules/yeoman-generator/lib/index.js:991:25)
   at /home/circleci/phovea/node_modules/run-async/index.js:49:25