rs-identify: don't construct std::string from nullptr, it's UB.
rs-identify: don't construct std::string from nullptr, it's UB.

Thanks to clang-tidy's bugprone-string-constructor.

LebedevRI/rawspeed,darktable-org/rawspeed,darktable-org/rawspeed,darktable-org/rawspeed,darktable-org/rawspeed,darktable-org/rawspeed,LebedevRI/rawspeed,LebedevRI/rawspeed,LebedevRI/rawspeed,LebedevRI/rawspeed