Parsing for bit types errors out

[VinaiRachakonda]

Repro

> create table mytable(pk int, val bit);
> insert into mytable values (1, b'');

panic: runtime error: index out of range [0] with length 0

goroutine 1 [running]:
github.com/dolthub/go-mysql-server/sql/parse.convertVal(0xc00071ea80, 0x2aece80, 0xc00071eb40, 0x0, 0x0)
	/Users/vinairachakonda/go/src/dolthub/go-mysql-server/sql/parse/parse.go:2441 +0x9b3
github.com/dolthub/go-mysql-server/sql/parse.ExprToExpression(0xc000140360, 0x2adb200, 0xc00071ea80, 0x2aece80, 0xc00071eb40, 0x0, 0x0)
	/Users/vinairachakonda/go/src/dolthub/go-mysql-server/sql/parse/parse.go:2185 +0x146e
github.com/dolthub/go-mysql-server/sql/parse.valuesToValues(0xc000140360, 0xc00071eac0, 0x1, 0x1, 0x0, 0xc0005b4b00, 0x107cc9c)
	/Users/vinairachakonda/go/src/dolthub/go-mysql-server/sql/parse/parse.go:1696 +0x1ba
github.com/dolthub/go-mysql-server/sql/parse.insertRowsToNode(0xc000140360, 0x2acd8c0, 0xc00071eae0, 0x0, 0x36eb540, 0x0, 0x0)
	/Users/vinairachakonda/go/src/dolthub/go-mysql-server/sql/parse/parse.go:1684 +0x189
github.com/dolthub/go-mysql-server/sql/parse.convertInsert(0xc000140360, 0xc0000ce8f0, 0xc0000ce8f0, 0x0, 0x0, 0xc0006aa900)
	/Users/vinairachakonda/go/src/dolthub/go-mysql-server/sql/parse/parse.go:1424 +0x108
github.com/dolthub/go-mysql-server/sql/parse.convert(0xc000140360, 0x2ac9ec0, 0xc0000ce8f0, 0xc0006aa930, 0x23, 0x0, 0xc0006aa930, 0x23, 0x0)
	/Users/vinairachakonda/go/src/dolthub/go-mysql-server/sql/parse/parse.go:180 +0xa56
github.com/dolthub/go-mysql-server/sql/parse.Parse(0xc000140360, 0xc0006aa930, 0x24, 0x0, 0x0, 0x0, 0x0)
	/Users/vinairachakonda/go/src/dolthub/go-mysql-server/sql/parse/parse.go:143 +0x5f7
github.com/dolthub/go-mysql-server.(*Engine).QueryNodeWithBindings(0xc0006e5290, 0xc0006ba510, 0xc0006aa930, 0x24, 0x0, 0x0, 0x0, 0x25c4e80, 0xc0000ce790, 0xc0005b4ff0, ...)
	/Users/vinairachakonda/go/src/dolthub/go-mysql-server/engine.go:144 +0x545
github.com/dolthub/go-mysql-server.(*Engine).QueryWithBindings(...)
	/Users/vinairachakonda/go/src/dolthub/go-mysql-server/engine.go:126
github.com/dolthub/go-mysql-server.(*Engine).Query(0xc0006e5290, 0xc0006ba510, 0xc0006aa930, 0x24, 0x0, 0x0, 0x0, 0x0, 0xc0000ce810, 0xc0000ce790, ...)
	/Users/vinairachakonda/go/src/dolthub/go-mysql-server/engine.go:117 +0x6a
github.com/dolthub/dolt/go/cmd/dolt/commands.(*sqlEngine).query(...)
	/Users/vinairachakonda/go/src/dolthub/dolt/go/cmd/dolt/commands/sql.go:1435
github.com/dolthub/dolt/go/cmd/dolt/commands.processQuery(0xc0006ba510, 0xc0006aa930, 0x24, 0xc0006be4c0, 0x0, 0x0, 0xffffffffffffffff, 0xc0006aa930, 0x24, 0x1010518, ...)
	/Users/vinairachakonda/go/src/dolthub/dolt/go/cmd/dolt/commands/sql.go:940 +0x245
github.com/dolthub/dolt/go/cmd/dolt/commands.runShell.func3(0xc000440d00)
	/Users/vinairachakonda/go/src/dolthub/dolt/go/cmd/dolt/commands/sql.go:720 +0x2b8
github.com/dolthub/ishell.handleUninterpretedInput(0xc00025e120, 0xc0006aa930, 0x24, 0x0, 0xc00007d201)
	/Users/vinairachakonda/go/pkg/mod/github.com/dolthub/ishell@v0.0.0-20210205014355-16a4ce758446/ishell.go:282 +0x146
github.com/dolthub/ishell.(*Shell).run(0xc00025e120)
	/Users/vinairachakonda/go/pkg/mod/github.com/dolthub/ishell@v0.0.0-20210205014355-16a4ce758446/ishell.go:242 +0x245
github.com/dolthub/ishell.(*Shell).Run(0xc00025e120)
	/Users/vinairachakonda/go/pkg/mod/github.com/dolthub/ishell@v0.0.0-20210205014355-16a4ce758446/ishell.go:136 +0x39
github.com/dolthub/dolt/go/cmd/dolt/commands.runShell(0xc0006ba510, 0xc0006be4c0, 0xc0006e4c90, 0xc0006e4f90, 0x0, 0xc0004a0a00)
	/Users/vinairachakonda/go/src/dolthub/dolt/go/cmd/dolt/commands/sql.go:743 +0x68c
github.com/dolthub/dolt/go/cmd/dolt/commands.execShell(0xc0006ba510, 0x4200100, 0xc0006e4c90, 0xc0006e4f90, 0x0, 0x35f2000, 0x0)
	/Users/vinairachakonda/go/src/dolthub/dolt/go/cmd/dolt/commands/sql.go:374 +0xe5
github.com/dolthub/dolt/go/cmd/dolt/commands.SqlCmd.Exec(0x28003ac, 0x6, 0x2ada780, 0xc0006bc300, 0xc0006b8258, 0x8, 0xc00000e090, 0x0, 0x0, 0xc000331290, ...)
	/Users/vinairachakonda/go/src/dolthub/dolt/go/cmd/dolt/commands/sql.go:341 +0x1637
github.com/dolthub/dolt/go/cmd/dolt/cli.SubCommandHandler.Exec(0x27fe6b8, 0x4, 0x2814d52, 0x11, 0xc0007d6780, 0x25, 0x25, 0x0, 0x2ada780, 0xc0006bc300, ...)
	/Users/vinairachakonda/go/src/dolthub/dolt/go/cmd/dolt/cli/command.go:165 +0x4bd
main.runMain(0x0)
	/Users/vinairachakonda/go/src/dolthub/dolt/go/cmd/dolt/dolt.go:280 +0x11a5
main.main()
	/Users/vinairachakonda/go/src/dolthub/dolt/go/cmd/dolt/dolt.go:115 +0x25