fix: report IMPORT_TRANSFORM of a non-existent domain instead of crashing

[SulimanAbdulrazzaq]

ValidateAndNormalizeConfig records the error IMPORT_TRANSFORM mentions non-existent domain "..." when the domain named by an IMPORT_TRANSFORM is not in the config, but it then calls importTransform with the nil domain anyway. That dereferences it, so dnscontrol check (and preview/push, which run the same validation) dies with a nil-pointer panic and a stack trace instead of printing the validation error.

The fix is the missing continue after the error is recorded, the same as the branch just above it that handles a transform table that cannot be decoded.

Example dnsconfig.js (the second domain names baz.com, which is not defined):

var REG_NONE = NewRegistrar("none");
var DNS_NONE = NewDnsProvider("none");
var TRANSFORM = [{ low: "1.2.3.10", high: "1.2.3.20", newBase: "123.123.123.100" }];
D("foo.com", REG_NONE, DnsProvider(DNS_NONE), A("one", "1.2.3.1"));
D("bar.com", REG_NONE, DnsProvider(DNS_NONE), A("www", "123.123.123.123"), IMPORT_TRANSFORM(TRANSFORM, "baz.com", 300));

dnscontrol check on the base commit (built on my fork's CI):

panic: runtime error: invalid memory address or nil pointer dereference
github.com/DNSControl/dnscontrol/v5/pkg/normalize.importTransform(0x0, ...)
	pkg/normalize/validate.go:286
github.com/DNSControl/dnscontrol/v5/pkg/normalize.ValidateAndNormalizeConfig(...)
	pkg/normalize/validate.go:534
exit status: 2

With this change:

1 Validation errors:
ERROR: IMPORT_TRANSFORM mentions non-existent domain "baz.com"
exiting due to validation errors
exit status: 1

Test: TestImportTransformMissingDomain in pkg/normalize/importTransform_test.go. It calls ValidateAndNormalizeConfig with an IMPORT_TRANSFORM that names a missing domain and expects the error (and no panic). On the base commit it fails with ValidateAndNormalizeConfig panicked: runtime error: invalid memory address or nil pointer dereference; with the change it passes.

Fork CI on the same tree as this PR (Go stable, linux): go test ./pkg/normalize/..., the full go test ./..., go build, go vet ./pkg/normalize/..., gofmt on pkg/normalize, go fix ./... (no diff) and golangci-lint all succeed. This change does not touch generated files.

Other places I checked: DNSConfig.FindDomain is only called here, and importTransform's other argument (domain) comes from the loop and is never nil.