Make sure externals is an array (#30466)
Make sure externals is an array (#30466)

Co-authored-by: Tobias Koppers <0cad4b845df78a4c17cdaf2dcf19cc8e39488037@googlemail.com>
azukaru/next.js,JeromeFitz/next.js,JeromeFitz/next.js,zeit/next.js,zeit/next.js,azukaru/next.js,azukaru/next.js,zeit/next.js,JeromeFitz/next.js,azukaru/next.js,JeromeFitz/next.js