wouldn't the code be creating $new_result ... even if we didn't need to return it?

The conditional operator works like if-then-else, so when the condition is true the clause following the : is not evaluated. Likewise for logical defined or: if the first expression is defined, the second expression (the one following //) is not evaluated.

But in either case $new_result would need to be computed within the relevant clause; otherwise, as Anonymous Monk says, the short-circuiting is removed. If the code (not shown) used to populate $new_result is too long to make this practical, then prefer the solutions proposed by Anonymous Monk or tobyink.