[mypyc] fix: inappropriate Nones in f-strings

if a variable is Final but the value is not yet known at compile-time, and that variable is used as an input to an f-string, the f-string will incorrectly contain "None"

Author: BobTheBuidler

mypyc/irbuild/specialize.py

@@ -719,7 +719,9 @@ def get_literal_str(expr: Expression) -> str | None:
             if isinstance(expr, StrExpr):
                 return expr.value
             elif isinstance(expr, RefExpr) and isinstance(expr.node, Var) and expr.node.is_final:
-                return str(expr.node.final_value)
+                final_value = expr.node.final_value
+                if final_value is not None:
+                    return str(final_value)
             return None
 
         for i in range(len(exprs) - 1):