func TestEmptySet(t *testing.T) {
valList := []*Validator{}
vals := NewValidatorSet(valList)
vals.IncrementProposerPriority(1)
}

and run:

=== RUN TestEmptySet
--- FAIL: TestEmptySet (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x30 pc=0x15b2b27]

goroutine 9 [running]:
testing.tRunner.func1(0xc000167200)
/usr/local/Cellar/go/1.11.2/libexec/src/testing/testing.go:792 +0x387
panic(0x168e260, 0x1de4340)
/usr/local/Cellar/go/1.11.2/libexec/src/runtime/panic.go:513 +0x1b9
github.com/tendermint/tendermint/types.(*ValidatorSet).incrementProposerPriority(0xc0004c72f0, 0x0)
/Users/ancaz/go/src/github.com/tendermint/tendermint/types/validator_set.go:146 +0xe7
github.com/tendermint/tendermint/types.(*ValidatorSet).IncrementProposerPriority(0xc0004c72f0, 0x1)
/Users/ancaz/go/src/github.com/tendermint/tendermint/types/validator_set.go:94 +0x71
github.com/tendermint/tendermint/types.TestEmptySet(0xc000167200)
/Users/ancaz/go/src/github.com/tendermint/tendermint/types/validator_set_test.go:606 +0x4a
testing.tRunner(0xc000167200, 0x18986d0)
/usr/local/Cellar/go/1.11.2/libexec/src/testing/testing.go:827 +0xbf
created by testing.(*T).Run
/usr/local/Cellar/go/1.11.2/libexec/src/testing/testing.go:878 +0x353

Process finished with exit code 1