modules: hal_nordic: cmake: Fix checking if uicr DT node is accessible

According to cmake documentation, in the `if(<string>)` expression
the string evaluates to false unless its value is one of the true
constants. Thus, the commands under `if(${uicr_path})` are never
executed. Use `if(DEFINED uicr_path)` instead, since `uicr_path`
is returned by `dt_nodelabel()` and it will be undefined if such
node does not exist.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>