Treat `allowed-repository-url-prefixes = []` the same as unspecified.

Previously, this would disallow all git clones except for those via
wildcard domains. This is highly unintuitive. It also meant that
disabling this function via environment variable was not possible.

conf/config.example.toml

···
 update-timeout = "60s"
 max-heap-size-ratio = 0.5 # * RAM_size
 forbidden-domains = []
-# allowed-repository-url-prefixes = <nil>
+allowed-repository-url-prefixes = []
 allowed-custom-headers = ["X-Clacks-Overhead"]
 
 [audit]

src/auth.go

···
 }
 
 func checkAllowedURLPrefix(repoURL string) error {
-	if config.Limits.AllowedRepositoryURLPrefixes != nil {
+	if len(config.Limits.AllowedRepositoryURLPrefixes) > 0 {
 		allowedPrefix := false
 		repoURL = strings.ToLower(repoURL)
 		for _, allowedRepoURLPrefix := range config.Limits.AllowedRepositoryURLPrefixes {
···
 		return auth, nil
 	}
 
-	if config.Limits.AllowedRepositoryURLPrefixes != nil {
+	if len(config.Limits.AllowedRepositoryURLPrefixes) > 0 {
 		causes = append(causes, AuthError{http.StatusUnauthorized, "DNS challenge not allowed"})
 	} else {
 		// DNS challenge gives absolute authority.

src/config.go

···
 	// List of domains unconditionally forbidden for uploads.
 	ForbiddenDomains []string `toml:"forbidden-domains" default:"[]"`
 	// List of allowed repository URL prefixes. Setting this option prohibits uploading archives.
-	AllowedRepositoryURLPrefixes []string `toml:"allowed-repository-url-prefixes"`
+	AllowedRepositoryURLPrefixes []string `toml:"allowed-repository-url-prefixes" default:"[]"`
 	// List of allowed custom headers. Header name must be in the MIME canonical form,
 	// e.g. `Foo-Bar`. Setting this option permits including this custom header in `_headers`,
 	// unless it is fundamentally unsafe.