Fix failure if package has no dependencies

[Nef10]

Fixes #25

[github-actions]

Coverage after merging empty into main

76.97%

Coverage Report

File	Stmts	Funcs	Lines	Uncovered Lines
Sources/SwiftDependencyUpdaterLibrary
Dependency.swift	52.81%	60%	51.90%	46–64
PackageDescription.swift	87.34%	100%	86.49%	56–61, 82–84, 94
ResolvedPackage.swift	87.69%	100%	86.44%	114, 23, 79–81, 89–91
SwiftDependencyUpdater.swift	100%	100%	100%
SwiftPackage.swift	95%	83.33%	95.95%	70, 76
SwiftPackageUpdate.swift	79.45%	100%	77.27%	41, 50, 54, 58–69
Update.swift	64.79%	75%	64.18%	41, 43–65
Sources/SwiftDependencyUpdaterLibrary/Extensions
Collection.swift	100%	100%	100%
String.swift	100%	100%	100%
Version.swift	0%	0%	0%	11–17, 19–32, 36–43
Sources/SwiftDependencyUpdaterLibrary/Services
Git.swift	88%	83.33%	88.89%	62–64, 71–74
GitHub.swift	71.88%	80%	71.19%	61, 64, 68–79, 84–86
Sources/SwiftDependencyUpdaterLibrary/SubCommands
GitHubCommand.swift	82.61%	37.50%	86.90%	48–53
ListCommand.swift	78.38%	20%	87.50%
UpdateCommand.swift	65.96%	20%	71.43%	22–27, 33–34