Fix zeroing of sparsevector when coefficient is MOI.ScalarAffineFunction #191

Job	Run time
Documentation	44s
Julia 1.6 - ubuntu-latest - x64	57s
Julia 1.6 - macOS-latest - x64	2m 42s
Julia 1.6 - windows-latest - x64	7m 59s
Julia 1 - ubuntu-latest - x64	1m 7s
Julia 1 - macOS-latest - x64	1m 22s
Julia 1 - windows-latest - x64	1m 22s
Julia nightly - ubuntu-latest - x64	1m 17s
Julia nightly - macOS-latest - x64	2m 4s
Julia nightly - windows-latest - x64	1m 47s
	21m 21s

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