fix(bls): fix is_infinity when aggregating onto empty AggregateSignature

[figtracer]

Issue

this bug happens because we are computing the flag is_infinity before branching, but the correct is_infinity value depends on the branch.

let's say that self is empty and we add an infinity signature (so other is AggSig::infinity()):

1. self.is_infinity starts as false
2. other.is_infinity is true
3. flag immediately updates to false && true = false incorrectly because we will then branch to else where we effectively add infinity + infinity; hence the flag should be true && true = true

to fix this:

1. we move the logical operator into the inner if statement: self.is_infinity flag is only set to true if both are points at infinity
2. we write self.is_infinity = other.is_infinity; if we branch to else, essentially we're adding infinity to whatever other is, so if other is infinity, self.is_infinity = trueelse self.is_infinity = false

Test Results

before the fix:

---- blst::empty_aggregate_plus_infinity_should_be_infinity stdout ----

thread 'blst::empty_aggregate_plus_infinity_should_be_infinity' (454378) panicked at crypto/bls/tests/tests.rs:545:5:
is_infinity flag should be true after adding infinity to empty
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

after:

test blst::empty_aggregate_plus_infinity_should_be_infinity ... ok
(...)

test result: ok. 38 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.11s

fixes #8491

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