Default XCASSET_FILES to nothing

Turns out that bash arrays are not initialized until they have a single
item. This means this variable is treated as unbound until we add an
item to it. By defaulting this to nothing, this check succeeds even if
no items are ever added to the array.

lib/cocoapods/generator/copy_resources_script.rb

@@ -195,7 +195,7 @@ EOS
 
       XCASSETS_COMPILE = <<EOS
 
-if [[ -n "${WRAPPER_EXTENSION}" ]] && [ "`xcrun --find actool`" ] && [ -n "$XCASSET_FILES" ]
+if [[ -n "${WRAPPER_EXTENSION}" ]] && [ "`xcrun --find actool`" ] && [ -n "${XCASSET_FILES:-}" ]
 then
   # Find all other xcassets (this unfortunately includes those of path pods and other targets).
   OTHER_XCASSETS=$(find "$PWD" -iname "*.xcassets" -type d)