Use folder name when metadata name is empty string or non-string

spec/fixtures/packages/package-with-invalid-activation-commands/package.json

@@ -1,5 +1,5 @@
 {
-  "name": "package-with-invalid-selectors",
+  "name": "package-with-invalid-commands",
   "version": "1.0.0",
   "activationCommands": {
     "<>": [

spec/fixtures/packages/package-with-main/package.cson

@@ -1 +1 @@
-'main': 'main-module.coffee'
+'main': 'main-module.coffee'

src/package.coffee

@@ -45,7 +45,8 @@ class Package
           throw error unless ignoreErrors
 
     metadata ?= {}
-    metadata.name ?= packageName
+    unless typeof metadata.name is 'string' and metadata.name.length > 0
+      metadata.name = packageName
 
     if includeDeprecatedAPIs and metadata.stylesheetMain?
       deprecate("Use the `mainStyleSheet` key instead of `stylesheetMain` in the `package.json` of `#{packageName}`", {packageName})