Avoid TypeError when mapper is undefined

[harigopal]

A ternary operation in the prism-custom-class.js file needs to be wrapped with parentheses for it to be evaluated properly. In it's current form, it's throwing a TypeError.

let a = "foo";
let b = undefined;
let c = "default";
a + b ? b() : c;
// Uncaught TypeError: b is not a function
a + (b ? b() : c);
// "foodefault"

[mAAdhaTTah]

Thanks for the PR! Make sure you run gulp to rebuild the files & commit them.

@RunDevelopment This seems like enough of an issue that we should consider cutting another release soon. Thoughts?

[RunDevelopment]

LGTM;

But please run our build process npm ci && npx gulp.

[RunDevelopment]

@mAAdhaTTah I agree. I'll get the changelog ready.

Maybe we could set up a tsconfig.json, so TSC can catch these null/undefined errors, so it won't happen again. After all, it's the second time this happened to me (#2106).