Ignore empty GAUCHE_LOAD_PATH

If GAUCHE_LOAD_PATH is empty string, string-split yielded ("") and
the empty string "" gets into *load-path*.  Although it is harmless,
we now make "" in *load-path* to trigger load-hook, so let's not allow
"" sneak into *load-path* inadvertently.  If the user wants to inject ""
via the enviornment variable, she can say GAUCHE_LOAD_PATH=":" etc.