scripts: edtlib: Fix broken 'required: true' check for booleans

Node._prop_val() returned too early for non-existent booleans, letting
missing 'required: true' booleans slip through without an error.

Fix it by rearranging the code to always do the 'required' check before
returning.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>