The ornamental design for a base for a light-emitting unit, as shown and described.
FIG. 1 is a perspective view of a base for a light-emitting unit showing our new design;
FIG. 2 is a rear view thereof;
FIG. 3 is a left side view thereof;
FIG. 4 is a top view thereof; and,
FIG. 5 is a bottom view thereof.
The broken line portions of the base for the light-emitting unit in all views represent unclaimed portions of the claimed design and form no part thereof.