The true solvent front does not have a sinusoidal shape. Perhaps you are looking at the pressure pulse made by the injector, which shows up as an artifact at the T zero time? That small "pulse" does indeed have a sinusoidal shape as it it caused by the making and breaking of pressure (the pressure change) which results from the injector valve switching between, in and out-of the fluid stream of mobile phase. Because this artifact is the result of a pressure disturbance or change in the system pressure (and flow), it will be observed at low sensitivity on the baseline (via UV wavelength or RI, or really most any detector).
If observed, any actual 'solvent front' peak shape for an unretained compound will not be sinusoidal, but look like a regular peak, at tzero, often with some tailing. If the solvent front shows overloading, then it may appear as a truncated, off-scale peak with tailing, but not sinusoidal.
If you want to learn more about HPLC principles and fundamentals, please consider reading one of the classic texts on the topic such as, "
Introduction to Modern Liquid Chromatography".
More information on "T zero" and Column Void Volume: "
Determination of HPLC Column Void Volume / Dead Volume, Dead Time (T zero)";
https://hplctips.blogspot.com/2011/05/d ... -time.html