MDEV-39122 Fix my_realpath to treat empty string as current directory

[mariadb-andrzejjarzabek]

The function my_realpath by default tries to pass its argument to the system function realpath, and attempts a fallback which doesn't resolve symlinks. The problem is my_realpath may be used to resolve symlinks in paths to files and directories that may not exist, for example the function error_if_data_home_dir() may be used to prevent creation of files in the server's data directory. For this reason it attempts to check the provided path's enclosing directory. The problem is when the provided path is a plain file name - interpreted as relative path to the file in the current directory. In this case the "enclosing directory" path will be an empty string, which to realpath() is an incorrect path - it fails with a "file not found" error. The fallback in my_realpath does resolve the empty string to the path of the current directory, but without resolving symlinks.

The fix is to change specification and implementation of my_realpath to explicitly treat an empty string argument as a path to the current directory, fixing not only error_if_data_home_dir() but also potential other similar usage patterns.

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