Dolt sql-server panics on unknown variable

[fulghum]

In Dolt version 1.12.0, when a dolt sql-server processes a request for an unknown user variable (e.g. select @foo;) the process panics and exits. This only seems to happen with a running sql server; not with the dolt sql shell.

Client repro:

mysql> select @foo;
ERROR 2013 (HY000): Lost connection to MySQL server during query
No connection. Trying to reconnect...
ERROR 2003 (HY000): Can't connect to MySQL server on 'localhost:3306' (61)
ERROR: 
Can't connect to the server

Server logs:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x58 pc=0x101f13e44]

goroutine 122 [running]:
github.com/dolthub/go-mysql-server/server.schemaToFields(0x140004c4ee8?, {0x1400007e8e8, 0x1, 0x10192a730?})
	github.com/dolthub/[email protected]/server/handler.go:607 +0xc4
github.com/dolthub/go-mysql-server/server.(*Handler).doQuery.func4()
	github.com/dolthub/[email protected]/server/handler.go:326 +0x110
golang.org/x/sync/errgroup.(*Group).Go.func1()
	golang.org/x/[email protected]/errgroup/errgroup.go:75 +0x58
created by golang.org/x/sync/errgroup.(*Group).Go in goroutine 21
	golang.org/x/[email protected]/errgroup/errgroup.go:72 +0x98

[fulghum]

Looks like column.Type is nil when vitess goes to send back the field/type metadata over the wire and causes this panic. Filling this in with types.Null seems to be enough to make it work correctly. I'll do a little more testing and tidying up and get a PR open for the fix.

This is another bug that only manifests when we go through the wire serialization code. @zachmu started work last week to get our existing tests to run over a sql-server connection, so very soon we should have protection against issues like this getting past our tests.

[fulghum]

Have a fix in place for user variables; looks like system variables have a similar crash, digging into that now.