VULTR provider: domain not created in web admin causes a panic on preview command

[birlorg]

Describe the bug
If you have not created the domain in the vultr DNS page, the dnscontrol preview command will crash(see additional context)

To Reproduce
create dnscontrol.js like below, run dnscontrol preview

Expected behavior
An error saying, hey dummy! You forgot to create the domain in the vutlr DNS panel (or alternatively, just create the domain).

DNS Provider

• VULTR

Additional context

dnscontrol.js:

var REG_NONE = NewRegistrar(""); // No registrar.
var DSP_VULTR = NewDnsProvider("vultr");

// Domains:

D("example.com", REG_NONE, DnsProvider(DSP_VULTR), A("@", "1.2.3.4"));

% dnscontrol preview
******************** Domain: example.com
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x0 pc=0x1058c682c]

goroutine 1 [running]:
github.com/StackExchange/dnscontrol/v4/providers/vultr.(*vultrProvider).GetZoneRecords(0x140009ba558, {0x14000c98c05, 0xb}, 0x1058daa6d?)
github.com/StackExchange/dnscontrol/v4/providers/vultr/vultrProvider.go:84 +0x8c
github.com/StackExchange/dnscontrol/v4/pkg/zonerecs.CorrectZoneRecords({0x12dd17a58, 0x140009ba558}, 0x14001014700)
github.com/StackExchange/dnscontrol/v4/pkg/zonerecs/zonerecords.go:12 +0x48
github.com/StackExchange/dnscontrol/v4/commands.run.func1(0x0?, 0x14000e9f798, 0x14000e9f728, {0x10617adb0, 0x106c4b980}, 0x0, 0x14000e9f6c6, 0x14000e9f6c8, {0x106161228, 0x140009ba570}, ...)
github.com/StackExchange/dnscontrol/v4/commands/previewPush.go:250 +0x530
github.com/StackExchange/dnscontrol/v4/commands.run({{{{0x1058d8a19, 0xc}, {0x0, 0x0}, 0x0, {{...}, {...}, 0x0, 0x0}}, {0x0, ...}}, ...}, ...)
github.com/StackExchange/dnscontrol/v4/commands/previewPush.go:290 +0x278
github.com/StackExchange/dnscontrol/v4/commands.Preview(...)
github.com/StackExchange/dnscontrol/v4/commands/previewPush.go:134
github.com/StackExchange/dnscontrol/v4/commands.init.func8.1(0x1400091ee00?)
github.com/StackExchange/dnscontrol/v4/commands/previewPush.go:32 +0x90
github.com/urfave/cli/v2.(*Command).Run(0x14000852000, 0x1400091ee00, {0x14000785b60, 0x1, 0x1})
github.com/urfave/cli/v2@v2.27.2/command.go:276 +0x71c
github.com/urfave/cli/v2.(*Command).Run(0x140007b42c0, 0x1400091ebc0, {0x140001c2000, 0x2, 0x2})
github.com/urfave/cli/v2@v2.27.2/command.go:269 +0x918
github.com/urfave/cli/v2.(*App).RunContext(0x1400091c200, {0x10616d790, 0x106cf3ec0}, {0x140001c2000, 0x2, 0x2})
github.com/urfave/cli/v2@v2.27.2/app.go:333 +0x514
github.com/urfave/cli/v2.(*App).Run(...)
github.com/urfave/cli/v2@v2.27.2/app.go:307
github.com/StackExchange/dnscontrol/v4/commands.Run({0x140000445c0?, 0x1058e5182?})
github.com/StackExchange/dnscontrol/v4/commands/commands.go:105 +0x444
main.main()
github.com/StackExchange/dnscontrol/v4/main.go:30 +0xbc

[tlimoncelli]

CC @pgaskin (maintainer of the VULTR provider)