[perl #55842] [BUG] empty .const .String broken

[François PERRAD via RT]

This bug starts with r28354 (cache string).
The cache don't handle empty string.

Now, in Pipp (PHP), an empty string is used to stringify boolean False.
// languages/pipp/src/pmc/phpboolean.pmc
    STRING* get_string() {
        if (PMC_int_val(SELF))
            return const_string(INTERP, "1");
        else
            return const_string(INTERP, "");
    }

Instead of an empty string, I obtain another cached string like "MD2",
"PhpUndef", ...