Only multiply in mul! for a non-zero alpha

[jishnub]

This follows the convention in LinearAlgebra and only evaluates A * b * alpha in mul! if alpha is non-zero.

Also, this improves performance when b is all zeros:

julia> using LinearAlgebra, FillArrays

julia> A = rand(100, 100);

julia> v = Zeros(100);

julia> y = similar(A, size(A,1));

julia> @btime mul!($y, $A, $v);
  1.437 μs (0 allocations: 0 bytes) # master
  14.799 ns (0 allocations: 0 bytes) # this PR

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.22%. Comparing base (0a58cd8) to head (193520f).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/fillalgebra.jl	75.00%	7 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #426       +/-   ##
===========================================
+ Coverage    0.00%   99.22%   +99.22%     
===========================================
  Files           8        8               
  Lines        1152     1168       +16     
===========================================
+ Hits            0     1159     +1159     
+ Misses       1152        9     -1143     

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