Nil pointer dereference for create user 'foo'@'%' identified by ''

[fulghum]

When specifying an empty string for a user password, Dolt panics with a nil pointer dereference:

2022-08-15T12:20:32-07:00 ERROR [conn %!d(<nil>)] mysql_server caught panic:
runtime error: invalid memory address or nil pointer dereference
runtime/panic.go:260 (0x102779e57)
runtime/signal_unix.go:835 (0x102792c97)
github.com/dolthub/go-mysql-server@v0.12.1-0.20220811192639-c0b3f448ca13/sql/plan/create_user.go:74 (0x103564d40)
github.com/dolthub/go-mysql-server@v0.12.1-0.20220811192639-c0b3f448ca13/sql/analyzer/resolve_variables.go:35 (0x10377834b)
github.com/dolthub/go-mysql-server@v0.12.1-0.20220811192639-c0b3f448ca13/sql/transform/node.go:304 (0x1034a02c3)
github.com/dolthub/go-mysql-server@v0.12.1-0.20220811192639-c0b3f448ca13/sql/analyzer/resolve_variables.go:34 (0x103778267)
github.com/dolthub/go-mysql-server@v0.12.1-0.20220811192639-c0b3f448ca13/sql/analyzer/batch.go:119 (0x10372d417)
github.com/dolthub/go-mysql-server@v0.12.1-0.20220811192639-c0b3f448ca13/sql/analyzer/batch.go:65 (0x10372cee7)
github.com/dolthub/go-mysql-server@v0.12.1-0.20220811192639-c0b3f448ca13/sql/analyzer/batch.go:56 (0x103721eff)
github.com/dolthub/go-mysql-server@v0.12.1-0.20220811192639-c0b3f448ca13/sql/analyzer/analyzer.go:524 (0x103721cd0)
github.com/dolthub/go-mysql-server@v0.12.1-0.20220811192639-c0b3f448ca13/sql/analyzer/analyzer.go:379 (0x103796777)
github.com/dolthub/go-mysql-server@v0.12.1-0.20220811192639-c0b3f448ca13/engine.go:329 (0x103796720)
github.com/dolthub/go-mysql-server@v0.12.1-0.20220811192639-c0b3f448ca13/engine.go:191 (0x103795b73)
github.com/dolthub/go-mysql-server@v0.12.1-0.20220811192639-c0b3f448ca13/server/handler.go:343 (0x1038103f3)
github.com/dolthub/go-mysql-server@v0.12.1-0.20220811192639-c0b3f448ca13/server/handler.go:607 (0x103812627)
github.com/dolthub/go-mysql-server@v0.12.1-0.20220811192639-c0b3f448ca13/server/handler.go:161 (0x10380e95b)
github.com/dolthub/vitess@v0.0.0-20220811180048-dec9dd483049/go/mysql/conn.go:1286 (0x102ece19f)
github.com/dolthub/vitess@v0.0.0-20220811180048-dec9dd483049/go/mysql/conn.go:930 (0x102ecb7df)
github.com/dolthub/vitess@v0.0.0-20220811180048-dec9dd483049/go/mysql/server.go:481 (0x102ee1b63)
runtime/asm_arm64.s:1165 (0x1027af993)

MySQL handles this gracefully and creates the user.