refactoring: simplified addition of "+0" in extract_constant

src/main/py/read_math_function.py

@@ -74,7 +74,7 @@ def simplify_easy_math_function(type, function):
 def extract_constant(function):
     coefficient, constant = function.split('x', 1) if 'x' in function else (function, '')
     if constant == "":
-        constant = constant + "+0"
+        constant += "+0"
     constant = constant.lstrip("+")
     return [constant, f"{coefficient}x"]