mrp: handle NaN elapsed time when calculating position

[seidnerj]

Problem

playing() raises ValueError: cannot convert float NaN to integer when a device
reports NaN as elapsed time, which makes the whole call fail:

File "pyatv/protocols/mrp/__init__.py", line 284, in build_playing_instance
    position=position(),
File "pyatv/protocols/mrp/__init__.py", line 226, in position
    return int(elapsed_time)
ValueError: cannot convert float NaN to integer

Seen in the wild via atvscript (as used by node-pyatv/homebridge-appletv-enhanced),
where the raised error terminates the process.

Cause

elapsedTime and elapsedTimeTimestamp are both double in ContentItemMetadata.proto
(lines 150 and 189), and devices do occasionally report NaN for them. position()
annotated elapsed_time as int and relied on ... or 0 as a fallback, but NaN is
truthy, so it goes straight into int() and raises. total_time() right above already
guards its (also double) duration field with math.isnan; position() never got the
same treatment.

Change

• Return None from position() when elapsedTime or elapsedTimeTimestamp is NaN,
  consistent with what total_time() does for a NaN duration. A NaN timestamp would
  otherwise raise the same ValueError from datetime.fromtimestamp() inside
  _cocoa_to_timestamp().
• Correct the elapsed_time annotation from int to float, matching the type the
  protobuf definition actually specifies.

Tests

Added test_metadata_position_nan, which fails on master with the traceback above and
passes with this change. Full test suite and chickn linting pass locally.