Let error message getter check for null pointer (#2634)
Let error message getter check for null pointer (#2634)

* check error message char pointer for null

* after a visit by clang-format-9

* return empty string in case of no error message (yet)
TileDB-Inc/TileDB,TileDB-Inc/TileDB,TileDB-Inc/TileDB,TileDB-Inc/TileDB