Land rapid7#7762, nil fix for generic/custom

Author: wvu

modules/payloads/singles/generic/custom.rb

@@ -42,10 +42,12 @@ def generate
       self.arch = actual_arch
     end
 
-    if datastore['PAYLOADFILE']
-      IO.read(datastore['PAYLOADFILE'])
-    elsif datastore['PAYLOADSTR']
+    if datastore['PAYLOADSTR']
       datastore['PAYLOADSTR']
+    elsif datastore['PAYLOADFILE']
+      IO.read(datastore['PAYLOADFILE'])
+    else
+      ''
     end
   end