build/xilinx/vivado: only generate constraints that are not empty

litex/build/xilinx/vivado.py

@@ -21,7 +21,7 @@ def _format_constraint(c):
     elif isinstance(c, Misc):
         return "set_property " + c.misc.replace("=", " ")
     elif isinstance(c, Inverted):
-        return ""
+        return None
     else:
         raise ValueError("unknown constraint {}".format(c))
 
@@ -33,6 +33,7 @@ def _format_xdc(signame, resname, *constraints):
         fmt_r += "." + resname[2]
     r = " ## {}\n".format(fmt_r)
     for c in fmt_c:
+        if c is not None:
             r += c + " [get_ports " + signame + "]\n"
     return r