Added a test case and error case

Author: mrz1836

private_key.go

@@ -3,13 +3,17 @@ package bitcoin
 import (
 	"crypto/ecdsa"
 	"encoding/hex"
+	"errors"
 	"math/big"
 
 	"github.com/bitcoinsv/bsvd/bsvec"
 )
 
 // PrivateKeyFromString turns a private key (hex encoded string) into an bsvec.PrivateKey
 func PrivateKeyFromString(privateKey string) (*bsvec.PrivateKey, error) {
+	if len(privateKey) == 0 {
+		return nil, errors.New("privateKey is missing")
+	}
 	privateKeyBytes, err := hex.DecodeString(privateKey)
 	if err != nil {
 		return nil, err

private_key_test.go

@@ -89,6 +89,7 @@ func TestPrivateKeyFromString(t *testing.T) {
 		{"E83385AF76B2B1997326B567461FB73DD9C27EAB9E1E86D26779F", "", true, true},
 		{"1234567", "", true, true},
 		{"0", "", true, true},
+		{"", "", true, true},
 	}
 
 	// Run tests