filter out /dev/null entries in list of patch files for PR

Ran into this issue when testing https://github.com/hpcugent/easybuild-easyconfigs/pull/1217, see https://github.com/hpcugent/easybuild-easyconfigs/pull/1217.diff
I think the `/dev/null` bit is specified to the diff format GitHub uses.

Example using the `diff` command line tool:

```diff
diff -ruN foo1 foo2
diff -ruN foo1/bar foo2/bar
--- foo1/bar    1970-01-01 01:00:00.000000000 +0100
+++ foo2/bar    2014-12-11 15:48:26.000000000 +0100
@@ -0,0 +1 @@
+foo
diff -ruN foo1/foo foo2/foo
--- foo1/foo    2014-12-11 15:48:24.000000000 +0100
+++ foo2/foo    1970-01-01 01:00:00.000000000 +0100
@@ -1 +0,0 @@
-foo
```