riviera-pro: handle empty macro (#681)

* Workaround for empty macro issue in Riviera-PRO
* Update rivierapro.py

Workaround for empty macro issue in Riviera-PRO - more elegant solution

vunit/sim_if/rivierapro.py

@@ -210,7 +210,9 @@ def compile_verilog_file_command(self, source_file):
         for include_dir in source_file.include_dirs:
             args += ["+incdir+%s" % include_dir]
         for key, value in source_file.defines.items():
-            args += ["+define+%s=%s" % (key, value)]
+            args += ["+define+%s" % key]
+            if value:
+                args[-1] += "=%s" % value
         return args
 
     def create_library(self, library_name, path, mapped_libraries=None):