Fix maintainer check condition

[damianstasik]

Summary

I've noticed that the value stored under is_core_team gets stringified, so some PRs may be mislabelled as coming from the core team.

I've checked this on my fork and saw stringified null returned by the is_core_team, and this PR explicitly checks for the correct value. Feel free to close this PR if you want to go with another approach.

How did you test this change?

Checked this change on my fork with and without listing myself in the maintainers file.

[react-sizebot]

Comparing: 61e713c...bb496f1

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.js	=	6.68 kB	6.68 kB	=	1.83 kB	1.83 kB
oss-stable/react-dom/cjs/react-dom-client.production.js	=	514.24 kB	514.24 kB	=	91.73 kB	91.73 kB
oss-experimental/react-dom/cjs/react-dom.production.js	=	6.69 kB	6.69 kB	=	1.83 kB	1.83 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js	=	556.18 kB	556.18 kB	=	98.72 kB	98.72 kB
facebook-www/ReactDOM-prod.classic.js	=	595.79 kB	595.79 kB	=	104.85 kB	104.85 kB
facebook-www/ReactDOM-prod.modern.js	=	586.21 kB	586.21 kB	=	103.30 kB	103.30 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against cac72e2

[poteto]

Oof thank you!