The ornamental design for a sensor, as shown and described.
FIG. 1 is a side perspective view of a sensor showing our new design;
FIG. 2 is a side view thereof, the opposite side being a mirror image of the side shown;
FIG. 3 is a front plan view thereof;
FIG. 4 is a rear view thereof; and,
FIG. 5 is an end view thereof, the opposite end being a mirror image of the end shown.
The broken line showing of structural features is included for the purpose of illustrating non-claimed subject matter and forms no part of the claimed design.