use === for String, String equality. (#27790)
* use === for String, String equality
* do pointer equality check before memory check
* remove whitespace
* Update base/strings/string.jl
Co-Authored-By: oxinabox <oxinabox@ucc.asn.au>
* Update base/strings/string.jl
Co-Authored-By: oxinabox <oxinabox@ucc.asn.au>