Change tuple[] repr to tuple[()]

[sobolevn]

Closes #15782

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

steam.py (https://github.com/Gobot1234/steam.py)
- steam/protobufs/struct_messages.py:24: error: Incompatible types in assignment (expression has type "tuple[]", variable has type "dict[str, Any]")  [assignment]
+ steam/protobufs/struct_messages.py:24: error: Incompatible types in assignment (expression has type "tuple[()]", variable has type "dict[str, Any]")  [assignment]

sublime_debugger (https://github.com/daveleroy/sublime_debugger)
- modules/ui/layout.py:264: error: Incompatible types in assignment (expression has type "tuple[Any, Any | int, Any | int, Any | int, Any, Any]", variable has type "tuple[]")  [assignment]
+ modules/ui/layout.py:264: error: Incompatible types in assignment (expression has type "tuple[Any, Any | int, Any | int, Any | int, Any, Any]", variable has type "tuple[()]")  [assignment]

discord.py (https://github.com/Rapptz/discord.py)
- discord/ext/tasks/__init__.py:497: error: Incompatible types in assignment (expression has type "tuple[]", variable has type "tuple[type[OSError], type[GatewayNotFound], type[ConnectionClosed], type[ClientError], type[TimeoutError]]")  [assignment]
+ discord/ext/tasks/__init__.py:497: error: Incompatible types in assignment (expression has type "tuple[()]", variable has type "tuple[type[OSError], type[GatewayNotFound], type[ConnectionClosed], type[ClientError], type[TimeoutError]]")  [assignment]

streamlit (https://github.com/streamlit/streamlit)
- lib/tests/streamlit/elements/date_input_test.py:174:16: error: Non-overlapping equality check (left operand type: "Union[date, Tuple[], Tuple[date], Tuple[date, date]]", right operand type: "List[datetime]")  [comparison-overlap]
+ lib/tests/streamlit/elements/date_input_test.py:174:16: error: Non-overlapping equality check (left operand type: "Union[date, Tuple[()], Tuple[date], Tuple[date, date]]", right operand type: "List[datetime]")  [comparison-overlap]

pydantic (https://github.com/samuelcolvin/pydantic)
- pydantic/v1/typing.py:47: error: Incompatible types in assignment (expression has type "tuple[]", variable has type "type[GenericAlias]")  [assignment]
+ pydantic/v1/typing.py:47: error: Incompatible types in assignment (expression has type "tuple[()]", variable has type "type[GenericAlias]")  [assignment]
- pydantic/v1/typing.py:53: error: Incompatible types in assignment (expression has type "tuple[]", variable has type "type[UnionType]")  [assignment]
+ pydantic/v1/typing.py:53: error: Incompatible types in assignment (expression has type "tuple[()]", variable has type "type[UnionType]")  [assignment]

[sobolevn]

Thanks! :)