basic: nulstr-util: add nulstr_get() returning the matching string

basic: nulstr-util: add nulstr_get() returning the matching string

Currently `nulstr_contains` returns a boolean, making it difficult to
identify which of the input strings matches the "needle".

Adding a new `nulstr_get()` function, returning a const pointer to the
matching string, eases this process and allows us to directly re-use the
result of a call to this function without additional processing or
memory allocation.

Signed-off-by: Arnaud Ferraris <arnaud.ferraris@collabora.com>