x/text/secure/precis: UsernameCaseMapped and UsernameCasePreserved should disallow empty usernames

[mitar]

Go version

go version go1.21.3 linux/amd64

What operating system and processor architecture are you using (go env)?

```shell GO111MODULE='' GOARCH='amd64' GOBIN='' GOCACHE='/home/mitar/.cache/go-build' GOENV='/home/mitar/.config/go/env' GOEXE='' GOEXPERIMENT='' GOFLAGS='' GOHOSTARCH='amd64' GOHOSTOS='linux' GOINSECURE='' GOMODCACHE='/home/mitar/.go/pkg/mod' GONOPROXY='' GONOSUMDB='' GOOS='linux' GOPATH='/home/mitar/.go' GOPRIVATE='' GOPROXY='https://proxy.golang.org,direct' GOROOT='/usr/local/go' GOSUMDB='sum.golang.org' GOTMPDIR='' GOTOOLCHAIN='auto' GOTOOLDIR='/usr/local/go/pkg/tool/linux_amd64' GOVCS='' GOVERSION='go1.21.3' GCCGO='/usr/bin/gccgo' GOAMD64='v1' AR='ar' CC='gcc' CXX='g++' CGO_ENABLED='1' GOMOD='/dev/null' GOWORK='' CGO_CFLAGS='-O2 -g' CGO_CPPFLAGS='' CGO_CXXFLAGS='-O2 -g' CGO_FFLAGS='-O2 -g' CGO_LDFLAGS='-O2 -g' PKG_CONFIG='pkg-config' GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build3082737491=/tmp/go-build -gno-record-gcc-switches' ```

What did you do?

Ran precis.UsernameCaseMapped.String("").

What did you expect to see?

An error returned as written in RFC 8265:

A username MUST NOT be zero bytes in length. This rule is to be enforced after any normalization and mapping of code points.

It seems DisallowEmpty option should be added to UsernameCaseMapped and UsernameCasePreserved. It is already added to Nickname and OpaqueString.

What did you see instead?

No error returned.