Fix: Remove nulls from context before evaluating expressions

[mapedraza]

This PR delete null elements from context. This fix arithmetic problems due to JEXL library bug:
#1440
TomFrost/Jexl#133

Overpasses:
#1471

[mapedraza]

From #1471

Conclusions:

• case b (v*3)
  • For Null values, is always failing (treating Null as 0)
  • For NaN values, it works well, giving Null as response.
• case f ((v*3)==Null?'soy null':'no soy null') is always failing
• Using explicitAttrs acts as sending a measurement with null value

This PR solves case b. Case f is still failing.

[AlvaroVega]

Not sure if this approach is introducing a bit of overhead, and maybe just only at measure level approach could be enough #1473
but LGTM