When you add sources and create empty macro like following:
VU.add_library("tb_uart_lib").add_source_files(SRC_PATH / "test" / "*.sv", defines={"EMPTY_MACRO" : ""})

It adds to vlog command a following switch:
vlog +define+EMPTY_MACRO=

However in Riviera-PRO it ends with following error:
ERROR VCP0501 "Missing option argument in command line. Option -dv requires an argument."

To workaround this you need to remove '=' operator when macro has no value.