False positive for Style/SafeNavigation when safe guarding arithmetic operation or assignment

[tiagotex]

Style/SafeNavigation returns false positives and tries to autocorrect when safeguarding a arithmetic operation or assignment on a chained method call.

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Expected behavior

Rubocop shouldn't consider this an offense, since we don't want to do the operation when object is null.

Actual behavior

Rubocop removes the safe guard and uses the safe operator.

Steps to reproduce the problem

foo.baz + bar if foo
foo.baz = bar if foo

Rubocop tries to fix to:

foo&.baz + bar
foo&.baz = bar

RuboCop version

$ rubocop -V
0.51.0 (using Parser 2.4.0.0, running on ruby 2.4.2 x86_64-darwin16)

[Drenmi]

Same issue as #5079. The problem seems to be that the cop is not checking that the last method in a method chain is a "dot-less" method (comparisons, arithmetics, hash access, etc.)

[tiagotex]

I'm working on a PR to fix it.

[tiagotex]

I fixed the problems described in this issue, but not the problem referenced in #5079, I might take a stab at it later.