The ornamental design for a modular wall system corner unit, as shown and described.
FIG. 1 is a front perspective view of a modular wall system corner unit showing my new design; and,
FIG. 2 is an end elevational view thereof.
The broken line showing of the adjacent structure in FIG. 1 is for environmental purposes only and forms no part of the claimed design.
Surface indicia, including wood graining, for example, that may be shown in the drawing figures is disclaimed as not being a part of the claimed ornamental design.