Fix parsing errors for nushell nightly (#112)

## Description

> Fixes the invalid type signature that is now caught by
[nushell/nushell#14510](nushell/nushell#14510)

nupm/publish.nu

@@ -176,7 +176,7 @@ export def main [
     }
 }
 
-def guess-url [] -> string {
+def guess-url []: nothing -> string {
     mut url = (do -i { ^git remote get-url origin | complete } | get stdout)
 
     if ($url | is-empty) {
@@ -191,7 +191,7 @@ def guess-url [] -> string {
     $url | str trim
 }
 
-def guess-revision [] -> string {
+def guess-revision []: nothing -> string {
     mut revision = (do -i { ^git describe --tags --abbrev=0 | complete }
         | get stdout)
 

nupm/status.nu

@@ -5,7 +5,7 @@ use utils/package.nu [open-package-file list-package-files]
 # Display status of a package
 export def main [
     path?: path  # path to the package
-] -> record<filename: string> {
+]: nothing -> record<filename: string> {
     let path = $path | default $env.PWD | path expand
 
     let pkg = open-package-file $path

nupm/utils/package.nu

@@ -33,7 +33,7 @@ export def open-package-file [dir: path] {
 # Lists files of a package
 #
 # This will be useful for file integrity checks
-export def list-package-files [pkg_dir: path, pkg: record] -> list<path> {
+export def list-package-files [pkg_dir: path, pkg: record]: nothing -> list<path> {
     let activation = match $pkg.type {
         'module' => $'use ($pkg.name)'
         'script' => {

nupm/utils/registry.nu

@@ -15,7 +15,7 @@ export def search-package [
     package: string  # Name of the package
     --registry: string  # Which registry to use (name or path)
     --exact-match  # Searched package name must match exactly
-] -> table {
+]: nothing -> table {
     let registries = if (not ($registry | is-empty)) and ($registry in $env.NUPM_REGISTRIES) {
         # If $registry is a valid column in $env.NUPM_REGISTRIES, use that
         { $registry : ($env.NUPM_REGISTRIES | get $registry) }