Allow call with empty argument list in strcat related functions (bug #44981) * libinterp/corefcn/strfns.cc (strvcat): return an empty string for an empty argument list. Simply dropping the input checking, will return an empty string. This makes it more consistent with cat(), vertcat, and horzcat() functions, which return [] for this cases. It makes it easier to support "strcat (cell{:})" when cell is empty. * scripts/strings/cstrcat.m, scripts/strings/strcat.m: same as above. But because [cellstr{:}] when cellstr is empty returns double ([]), we specifically return "".