Enable archiving of Strings with hash 0

Currently Strings with hash 0 are excluded from CDS heap archiving, since a call to String::hashCode() might store a value to it. This blocks adding archiving in places where storing "" is needed, since the archived empty string will not share identity with the "" literal.

JDK-8221723 seek to make sure that we do not store a zero value unnecessarily to String.hash, meaning any String added to the archive (where we guarantee hash has been calculated) will not observe any stores at runtime, thus we should remove the special-casing of such strings.