test: fs_helper: Skip empty srcdir when creating a filesystem

FsHelper.mk_fs() always creates a srcdir via setup(), then passes it
to the module-level mk_fs(). This fails for filesystem types like ext2
that do not support the -d flag, raising ValueError even when no files
need to be copied.

Pass None for src_dir when the srcdir is empty, so that creating an
empty filesystem works for all supported types.

Signed-off-by: Simon Glass <[email protected]>

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