Can't that way... xlsread imports what it finds as numeric and an empty cell is not numeric. You could use the multiple optional returns and read the raw cell that (I think, untested) will return an empty cell.
Probably the easiest kludge to fix your problem is
Test_files = dir([SourceFolder '*.xlsx']);
No_of_Tests = size(Test_files,1);
for n = 1:No_of_Tests
X(n,:) = xlsread(Test_files(n).name,'TEST','D101');
But, per the doc xlsread is deprecated, anyways; use one of the suggested alternates (altho even there you may have trouble with a single empty cell expecting numeric result).