qemu-xilinx-system-native: Fix license check when empty LICENSE_FLAGS_ACCEPTED

Fix an issue that results in:

 Traceback (most recent call last):
  File "Var <DEPENDS>", line 1, in <module>
  bb.data_smart.ExpansionError: Failure expanding variable DEPENDS, expression was pkgconfig-native  glib-2.0-native zlib-native ninja-native meson-native pixman-native qemu-xilinx-native bison-native ninja-native meson-native qemu-xilinx-multiarch-helper-native${@' pmu-rom-native' if 'xilinx' in d.getVar('LICENSE_FLAGS_ACCEPTED').split() else ''} alsa-lib-native dtc-native libgcrypt-native libslirp-native which triggered exception AttributeError: 'NoneType' object has no attribute 'split'
  The variable dependency chain for the failure is: DEPENDS

This only occurs if no LICENSE_FLAGS_ACCEPTED is defined.  This should not be
a blocking error.  Instead the user is warned of reduced functionality and we
continue the build.

Signed-off-by: Mark Hatle <mark.hatle@amd.com>

Author: mhatle

meta-xilinx-core/recipes-devtools/qemu/qemu-xilinx-system-native_%.bbappend

@@ -1,2 +1,2 @@
 # Automatically enable pmu-rom-native for ZynqMP support, or warn the user
-DEPENDS .= "${@' pmu-rom-native' if 'xilinx' in d.getVar('LICENSE_FLAGS_ACCEPTED').split() else ''}"
+DEPENDS .= "${@' pmu-rom-native' if 'xilinx' in (d.getVar('LICENSE_FLAGS_ACCEPTED') or '').split() else ''}"