I claim the ornamental design for a ball whisk, as shown and described.
FIG. 1 is an isometric view of a ball whisk;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The ornamental design which is claimed is shown in solid lines in the drawings. The broken line showing of portions of the ball whisk illustrate portions of the ball whisk which form no part of the claimed design.