The ornamental design for a nightlight, as shown and described.
FIG. 1 is a front elevational view of a night light showing my new design;
FIG. 2 is a left elevational view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a bottom plan view thereof;
FIG. 5 is a front perspective view thereof;
FIG. 6 is a right elevational view thereof; and,
FIG. 7 is a rear elevational view thereof.
The structural features depicted by broken lines have been shown for the purpose of illustrating portions of the night light that form no part of the claimed design.