fix: golangci-lint failing due nil check (#333)

Added the fix and introduced `make lint` for local tests

dsse/verify.go:98:6: S1009: should omit nil check; len() for []byte is defined as zero (gosimple)
                if sig.Certificate != nil && len(sig.Certificate) > 0 {
                   ^

Signed-off-by: Kairo Araujo <kairo.araujo@testifysec.com>

Makefile

@@ -29,3 +29,8 @@ schema: ## Generate the attestor schema json files
 
 help: ## Display this help screen
 	@grep -h -E '^[a-zA-Z_-]+:.*?## .*$$' $(MAKEFILE_LIST) | awk 'BEGIN {FS = ":.*?## "}; {printf "\033[36m%-30s\033[0m %s\n", $$1, $$2}'
+
+lint: ## Run the linter
+	@golangci-lint run
+	@go fmt ./...
+	@go vet ./...

dsse/verify.go

@@ -95,7 +95,7 @@ func (e Envelope) Verify(opts ...VerificationOption) ([]CheckedVerifier, error)
 	checkedVerifiers := make([]CheckedVerifier, 0)
 	verified := 0
 	for _, sig := range e.Signatures {
-		if sig.Certificate != nil && len(sig.Certificate) > 0 {
+		if len(sig.Certificate) > 0 {
 			cert, err := cryptoutil.TryParseCertificate(sig.Certificate)
 			if err != nil {
 				continue