Fix the crash of ConverterTest.PredictSetKey in GYP debug build.
Fix the crash of ConverterTest.PredictSetKey in GYP debug build.

The crash was caused by trying to output nullptr into the log stream. This CL
employed std::optional to represent maybe-present string. Besides, implement
the minimal StubPredictor::Predict so that the output segments meets the
criteria of IsValidSegment() in conveter.cc.

PiperOrigin-RevId: 480622359

fcitx/mozc,google/mozc,google/mozc,fcitx/mozc,google/mozc,google/mozc,fcitx/mozc,fcitx/mozc,google/mozc,fcitx/mozc