Fix type errors in orm.code modules

[chrisjsewell]

If the orm code modules are removed from the pre-commit mypy excluded list, below are the current errors.

1. Issues assuming that a computer has been set
2. Issues about what type is used for the executable path (see also Add support for pathlib.Path in the Transport interface #5797 (comment))

It would be good to get these fixed

mypy.....................................................................Failed
- hook id: mypy
- exit code: 1

aiida/orm/nodes/data/code/portable.py:57: error: Incompatible types in assignment (expression has type "str", variable has type "PurePath")  [assignment]
aiida/orm/nodes/data/code/portable.py:58: error: Argument 1 to "put_object_from_tree" of "NodeRepository" has incompatible type "Path"; expected "str"  [arg-type]
aiida/orm/nodes/data/code/portable.py:94: error: Incompatible return value type (got "PurePath", expected "Path")  [return-value]
aiida/orm/nodes/data/code/installed.py:45: error: Incompatible types in assignment (expression has type "str", variable has type "PurePath")  [assignment]
aiida/orm/nodes/data/code/installed.py:74: error: Item "None" of "Optional[Computer]" has no attribute "get_transport"  [union-attr]
aiida/orm/nodes/data/code/installed.py:93: error: Item "None" of "Optional[Computer]" has no attribute "pk"  [union-attr]
aiida/orm/nodes/data/code/installed.py:100: error: Incompatible return value type (got "PurePath", expected "Path")  [return-value]
aiida/orm/nodes/data/code/installed.py:111: error: Item "None" of "Optional[Computer]" has no attribute "label"  [union-attr]
aiida/orm/nodes/data/code/legacy.py:144: error: Item "None" of "Optional[Computer]" has no attribute "label"  [union-attr]
aiida/orm/nodes/data/code/legacy.py:152: error: Item "None" of "Optional[Computer]" has no attribute "label"  [union-attr]
aiida/orm/nodes/data/code/legacy.py:160: error: Item "None" of "Optional[Computer]" has no attribute "label"  [union-attr]
aiida/orm/nodes/data/code/legacy.py:169: error: Item "None" of "Optional[Computer]" has no attribute "label"  [union-attr]
aiida/orm/nodes/data/code/legacy.py:208: error: Item "List[Any]" of "Union[List[Any], Any]" has no attribute "pk"  [union-attr]
aiida/orm/nodes/data/code/legacy.py:212: error: Value of type "Optional[List[Any]]" is not indexable  [index]
aiida/orm/nodes/data/code/legacy.py:306: error: Item "List[Any]" of "Union[List[Any], Any]" has no attribute "label"  [union-attr]
aiida/orm/nodes/data/code/legacy.py:308: error: Item "List[Any]" of "Union[List[Any], Any]" has no attribute "pk"  [union-attr]
aiida/orm/nodes/data/code/legacy.py:352: error: Item "None" of "Optional[Computer]" has no attribute "get_transport"  [union-attr]
Found 17 errors in 3 files (checked 728 source files)