Fix mod check

.gitignore

@@ -7,3 +7,4 @@ __pycache__
 .vscode
 .coverage
 .hypothesis
+.direnv

roll/cli/__init__.py

@@ -87,6 +87,6 @@ def _throw(roll: Roll) -> Throw:
     throw = roll.throw()
     for i, result in enumerate(throw.results, start=1):
         click.echo(f"{i}:\t|{result: >4}")
-    if roll.modifier is not None:
+    if roll.modifier:
         click.echo(f"mod:\t{roll.modifier_str(): >5}")
     return throw