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

Co-authored-by: axe312ger <[email protected]>
Co-authored-by: Lennart <[email protected]>
Co-authored-by: Ward Peeters <[email protected]>
(cherry picked from commit 7bc7cf6)

Co-authored-by: Xavi Ivars <[email protected]>

Author: gatsbybot