[PG adapter] Use infinity const

Kinda microoptimization, but why not?

```ruby
require 'benchmark'

n = 10_000_000

Benchmark.bm do |x|
  x.report('/') { n.times { 1.0 / 0.0 } }
  x.report('const') { n.times { Float::INFINITY } }
end
```

```
       user     system      total        real
/  0.920000   0.010000   0.930000 (  0.913245)
const  0.500000   0.000000   0.500000 (  0.506287)
```