fix: In Parsing expandVariables return unchanged value if not exists in envMap or os-environ

[vaibhavmunjal]

• [x] BUG: Variable not correct (https://github.com/joho/godotenv/issues/127)

With regex expression (\\)?(\$)(\()?\{?([A-Z0-9_]+)?\}? https://regex101.com/r/IM10C6/1 any value after $ is treating as variable, Variable is checked in envMap and return empty string if not found, causing some values to cut down e.g. https://github.com/joho/godotenv/issues/127#issuecomment-787426255

[luisdavim]

This is pretty much a duplicate of #72