Make kernel(::SMat) better infereable

[lgoettgens]

Without this change, the compiler only infers Union{dense_matrix_type(C), Nothing}, as it does not know that the if/elseif clause get always taken due to the check in the beginning. Adding this explicit error removes Nothing from the possible return types.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.81%. Comparing base (a024dc6) to head (f0ecd81).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1633      +/-   ##
==========================================
- Coverage   75.85%   75.81%   -0.05%     
==========================================
  Files         361      361              
  Lines      113702   113701       -1     
==========================================
- Hits        86249    86201      -48     
- Misses      27453    27500      +47     

Files with missing lines	Coverage Δ
src/Sparse/Rref.jl	99.07% <100.00%> (-0.01%)	⬇️

... and 24 files with indirect coverage changes