Do not assert if XRAY_PART env variable is not present

Signed-off-by: Alessandro Comodi <acomodi@antmicro.com>

prjxray/segmaker.py

@@ -80,6 +80,7 @@ class Segmaker:
         self.part = part
         if self.part is None:
             self.part = util.get_part()
+            assert self.part, "No part specified."
 
         self.verbose = verbose if verbose is not None else os.getenv(
             'VERBOSE', 'N') == 'Y'

prjxray/util.py

@@ -17,7 +17,6 @@ def get_db_root():
 
 def get_part():
     ret = os.getenv("XRAY_PART", None)
-    assert ret
 
     return ret