JobSpawn crashes when passed `null` for `onExit`

[redfire75369]

Description of the problem or steps to reproduce

Create a plugin with shell.JobSpawn that takes in nil as onExit

shell.JobSpawn("curl", args, nil, nil, nil)

Micro crashes with

Micro encountered an error: runtime.errorString runtime error: invalid memory address or nil pointer dereference
runtime/panic.go:260 (0x24ecd6)
runtime/panic.go:259 (0x24eca6)
github.com/zyedidia/micro/v2/cmd/micro/micro.go:423 (0x6e566b)
github.com/zyedidia/micro/v2/cmd/micro/micro.go:400 (0x6e5251)
runtime/proc.go:250 (0x23c95e)
runtime/asm_amd64.s:1594 (0x26a0c1)

If you can reproduce this error, please report it at https://github.com/zyedidia/micro/issues

Specifications

Version: 2.0.14
Commit hash: 04c5770
OS: Windows
Terminal: Windows Terminal/Command Prompt

[JoeKar]

Already fixed with #3554.