`juju show-task ""` panics

[Simon Déziel]

Providing a bogus task ID causes a panic:

```
# juju show-task ""
panic: invalid action id ""

goroutine 1 [running]:
github.com/juju/names/v4.NewActionTag({0x7ffe8aac24b0, 0x0})
 /build/snapcraft-juju-67735438bf012563f9fb9dbad398e9fd/parts/juju/build/vendor/github.com/juju/names/v4/action.go:36 +0x125
github.com/juju/juju/api/client/action.(*Client).Actions(0xc0000cc9c0, {0xc000b80e20, 0x1, 0xc000dc7488?})
 /build/snapcraft-juju-67735438bf012563f9fb9dbad398e9fd/parts/juju/build/api/client/action/client.go:35 +0x94
github.com/juju/juju/cmd/juju/action.fetchResult({0x50f64d0, 0xc0000cc9c0}, {0x7ffe8aac24b0, 0x0})
 /build/snapcraft-juju-67735438bf012563f9fb9dbad398e9fd/parts/juju/build/cmd/juju/action/common.go:486 +0xf3
github.com/juju/juju/cmd/juju/action.(*showTaskCommand).Run(0xc0009257a0, 0xc0006b5bc0)
 /build/snapcraft-juju-67735438bf012563f9fb9dbad398e9fd/parts/juju/build/cmd/juju/action/showtask.go:162 +0x4e5
github.com/juju/juju/cmd/modelcmd.(*modelCommandWrapper).Run(0xc0006154a0, 0xc000c7f950?)
 /build/snapcraft-juju-67735438bf012563f9fb9dbad398e9fd/parts/juju/build/cmd/modelcmd/modelcommand.go:663 +0x123
github.com/juju/juju/cmd/modelcmd.(*baseCommandWrapper).Run(0xc0008dcfe0, 0x400000005?)
 /build/snapcraft-juju-67735438bf012563f9fb9dbad398e9fd/parts/juju/build/cmd/modelcmd/base.go:554 +0xaf
github.com/juju/cmd/v3.(*SuperCommand).Run(0xc0002492c0, 0xc0006b5bc0)
 /build/snapcraft-juju-67735438bf012563f9fb9dbad398e9fd/parts/juju/build/vendor/github.com/juju/cmd/v3/supercommand.go:534 +0x378
github.com/juju/cmd/v3.Main({0x50eb738, 0xc0002492c0}, 0xc0006b5bc0, {0xc0003b8c60, 0x2, 0x2})
 /build/snapcraft-juju-67735438bf012563f9fb9dbad398e9fd/parts/juju/build/vendor/github.com/juju/cmd/v3/cmd.go:419 +0x25d
github.com/juju/juju/cmd/juju/commands.jujuMain.Run({0xc0000e2140?}, {0xc0000720f0, 0x3, 0x3})
 /build/snapcraft-juju-67735438bf012563f9fb9dbad398e9fd/parts/juju/build/cmd/juju/commands/main.go:202 +0x8a5
github.com/juju/juju/cmd/juju/commands.Main(...)
 /build/snapcraft-juju-67735438bf012563f9fb9dbad398e9fd/parts/juju/build/cmd/juju/commands/main.go:125
main.main()
 /build/snapcraft-juju-67735438bf012563f9fb9dbad398e9fd/parts/juju/build/cmd/juju/main.go:27 +0x72
```

Additional information:
# snap list juju
Name Version Rev Tracking Publisher Notes
juju 3.2.0 23190 latest/stable canonical✓ -

[Heather Lanigan (hmlanigan)]

3.1: https://github.com/juju/juju/pull/15802