Make latch_map_file truly optional (#869)

Right now if a technology doesn't specify the latch map file, "techmap
-map None" is written into the generated tcl.

hammer/synthesis/yosys/__init__.py

@@ -249,7 +249,7 @@ def init_environment(self) -> bool:
 
     def syn_generic(self) -> bool:
         # TODO: is there a better way to do this? like self.get_setting()
-        if self._database.has_setting("synthesis.yosys.latch_map_file"):
+        if self._database.has_setting("synthesis.yosys.latch_map_file") and self.get_setting('synthesis.yosys.latch_map_file') is not None:
             latch_map = f"techmap -map {self.get_setting('synthesis.yosys.latch_map_file')}"
         else:  # TODO: make the else case better
             latch_map = ""