[OpenZeppelin audit N-07] replace or_insert_with with or_default for BTreeMap and HashMap

[silathdiir]

Description

Reference N-07:

The codebase contains some instances where the HashMap method or_insert_with is being used to construct default values. Some examples:
Line 73
Line 92
For cleaner and more efficient code, consider replacing instances of or_insert_with that are used for simple default value initialization with the more concise or_default() method.