fix: fix package spec parsing during cache add process

Close #1734

PR-URL: #1738
Credit: @mjeanroy
Close: #1738
Reviewed-by: @isaacs

EDIT(@isaacs): Removed test, pending full test coverage of lib/cache.js

Author: mjeanroy

lib/cache.js

@@ -80,7 +80,7 @@ const add = async (args, where) => {
     '    npm cache add <folder>\n'
   log.silly('cache add', 'args', args)
   const spec = args[0] +
-    (args[1] === undefined || args[1] === null ? `@${args[1]}` : '')
+    (args[1] === undefined || args[1] === null ? '' : `@${args[1]}`)
 
   log.verbose('cache add', 'spec', spec)
   if (!spec) {