nir: Don't copy empty array

nir: Don't copy empty array

It's undefined behavior UBSAN complains about, so fixing this will
reduce the noise a bit.

../src/compiler/nir/nir_clone.c:710:4: runtime error: null pointer passed as argument 2, which is declared to never be null"}
    #0 0xac781be4 in clone_function ../src/compiler/nir/nir_clone.c:710"}
    #1 0xac781be4 in nir_shader_clone ../src/compiler/nir/nir_clone.c:740"}
    #2 0xacf99442 in panfrost_shader_compile ../src/gallium/drivers/panfrost/pan_assemble.c:54"}
    #3 0xacf6b268 in panfrost_bind_shader_state ../src/gallium/drivers/panfrost/pan_context.c:1960"}
    #4 0xaae326bc in set_fragment_shader ../src/mesa/state_tracker/st_cb_clear.c:135"}
    #5 0xaae326bc in clear_with_quad ../src/mesa/state_tracker/st_cb_clear.c:335"}
    #6 0xaae326bc in st_Clear ../src/mesa/state_tracker/st_cb_clear.c:518"}
    #7 0x494d0e in deqp::gles2::TestCaseWrapper::iterate(tcu::TestCase*) (/deqp/modules/gles2/deqp-gles2+0x2ad0e)"}
    #8 0x7f9cf2 in tcu::TestSessionExecutor::iterateTestCase(tcu::TestCase*) (/deqp/modules/gles2/deqp-gles2+0x38fcf2)"}
    #9 0x7fa5f0 in tcu::TestSessionExecutor::iterate() (/deqp/modules/gles2/deqp-gles2+0x3905f0)"}
    #10 0x7e1aac in tcu::App::iterate() (/deqp/modules/gles2/deqp-gles2+0x377aac)"}
    #11 0x492d4c in main (/deqp/modules/gles2/deqp-gles2+0x28d4c)"}
    #12 0xb64b9aa8 in __libc_start_main (/lib/arm-linux-gnueabihf/libc.so.6+0x1aaa8)"}

Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>