fix(gatsby-source-contentful): Prevent null pointer exception (#35244) (#35492)

Co-authored-by: axe312ger <opensource@axe312.dev>
Co-authored-by: Lennart <lekoarts@gmail.com>
Co-authored-by: Ward Peeters <ward@coding-tech.com>
(cherry picked from commit 7bc7cf6)

Co-authored-by: Xavi Ivars <xivarsribes@vistaprint.com>

Author: gatsbybot