Fix my_realpath to treat empty string as current directory

When a plain filename is passed to error_if_data_home_dir() and the current working directory path contains symlinks, the function fails to detect that the path to be checked is in fact inside the data directory. In the typical scenario the relative path is interpreted as relative to the current working directory, and the current working directory is the data directory.

Note: original title and description were a result of misdiagnosis. The function error_if_data_home_dir uses the path to data directory with resolved symlinks. The part that was problematic was the path to the directory in which the backup directory was to be created.