[BugFix][C++] Check is not nullptr before calling ToString and fix empty prefix bug

[acezen]

Proposed changes

see issue #338 #340
and one more:
GraphInfo is invalid when prefix is empty, since the prefix of graph info is the begin prefix of all paths, it should be empty and no default value.

Checklist

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Further comments

close #338
close #340

[acezen]

@lixueclaire , this Pull Request needs your review~