[#1183] Properly cache the 'generic project type

Before this chance if the project type was 'generic there was project
type caching.

For the issue to be solved completely we'll need to introduce
something similar to the projectile-cached-project-name, though.

projectile.el

@@ -2394,28 +2394,31 @@ Normally you'd set this from .dir-locals.el.")
 (put 'projectile-project-type 'safe-local-variable #'symbolp)
 
 (defun projectile-detect-project-type ()
-  "Detect the type of the current project."
-  (let ((project-type (cl-find-if
-                       (lambda (project-type)
-                         (let ((marker (plist-get (gethash project-type projectile-project-types) 'marker-files)))
-                           (if (listp marker)
-                               (and (projectile-verify-files marker) project-type)
-                             (and (funcall marker) project-type))))
-                       (projectile-hash-keys projectile-project-types))))
-    (when project-type
-      (puthash (projectile-project-root) project-type projectile-project-type-cache))
+  "Detect the type of the current project.
+Fallsback to a generic project type when the type can't be determined."
+  (let ((project-type (or (cl-find-if
+                           (lambda (project-type)
+                             (let ((marker (plist-get (gethash project-type projectile-project-types) 'marker-files)))
+                               (if (listp marker)
+                                   (and (projectile-verify-files marker) project-type)
+                                 (and (funcall marker) project-type))))
+                           (projectile-hash-keys projectile-project-types))
+                          'generic)))
+    (puthash (projectile-project-root) project-type projectile-project-type-cache)
     project-type))
 
 (defun projectile-project-type ()
-  "Determine the project's type based on its structure."
+  "Determine the project's type based on its structure.
+
+The project type is cached for improved performance."
   (if projectile-project-type
       projectile-project-type
     (let ((project-root (ignore-errors (projectile-project-root))))
       (if project-root
           (or (gethash project-root projectile-project-type-cache)
-              (projectile-detect-project-type)
-              'generic)
-        'generic))))
+              (projectile-detect-project-type))
+        ;; if we're not in a project we just return nil
+        nil))))
 
 ;;;###autoload
 (defun projectile-project-info ()