execution: fix min/max aggregators

[MichaHoffmann]

without the fix:

=== RUN   TestInstantQuery/disableOptimizers=true/fuzz_-_min_with_Inf#02
=== RUN   TestInstantQuery/disableOptimizers=true/fuzz_-_min_with_Inf#02/disableFallback=false
    testutil.go:91: engine_test.go:3868: ""

        	exp: &promql.Result{Err:error(nil), Value:promql.Vector{promql.Sample{T:50000, F:9.223372036854776e+18, H:(*histogram.FloatHistogram)(nil), Metric:labels.Labels{labels.Label{Name:"pod", Value:"nginx-1"}, labels.Label{Name:"route", Value:"/"}}}, promql.Sample{T:50000, F:9.223372036854776e+18, H:(*histogram.FloatHistogram)(nil), Metric:labels.Labels{labels.Label{Name:"pod", Value:"nginx-2"}, labels.Label{Name:"route", Value:"/"}}}}, Warnings:annotations.Annotations(nil)}

        	got: &promql.Result{Err:error(nil), Value:promql.Vector{promql.Sample{T:50000, F:+Inf, H:(*histogram.FloatHistogram)(nil), Metric:labels.Labels{labels.Label{Name:"pod", Value:"nginx-1"}, labels.Label{Name:"route", Value:"/"}}}, promql.Sample{T:50000, F:+Inf, H:(*histogram.FloatHistogram)(nil), Metric:labels.Labels{labels.Label{Name:"pod", Value:"nginx-2"}, labels.Label{Name:"route", Value:"/"}}}}, Warnings:annotations.Annotations{}}

        Diff:
        --- Expected
        +++ Actual
        @@ -1,3 +1,3 @@
        -(*promql.Result)({pod="nginx-1", route="/"} => 9223372036854776000 @[50000]
        -{pod="nginx-2", route="/"} => 9223372036854776000 @[50000])
        +(*promql.Result)({pod="nginx-1", route="/"} => +Inf @[50000]
        +{pod="nginx-2", route="/"} => +Inf @[50000])



=== RUN   TestInstantQuery/disableOptimizers=true/fuzz_-_min_with_NaN#03
=== RUN   TestInstantQuery/disableOptimizers=true/fuzz_-_min_with_NaN#03/disableFallback=false
    testutil.go:91: engine_test.go:3868: ""

        	exp: &promql.Result{Err:error(nil), Value:promql.Vector{}, Warnings:annotations.Annotations(nil)}

        	got: &promql.Result{Err:error(nil), Value:promql.Vector{promql.Sample{T:50000, F:NaN, H:(*histogram.FloatHistogram)(nil), Metric:labels.Labels{labels.Label{Name:"pod", Value:"nginx-1"}, labels.Label{Name:"route", Value:"/"}}}, promql.Sample{T:50000, F:NaN, H:(*histogram.FloatHistogram)(nil), Metric:labels.Labels{labels.Label{Name:"pod", Value:"nginx-2"}, labels.Label{Name:"route", Value:"/"}}}}, Warnings:annotations.Annotations{}}

        Diff:
        --- Expected
        +++ Actual
        @@ -1,2 +1,3 @@
        -(*promql.Result)()
        +(*promql.Result)({pod="nginx-1", route="/"} => NaN @[50000]
        +{pod="nginx-2", route="/"} => NaN @[50000])