Zero is ignored by Environment key provider

Problem/Motivation

Setting MY_TOGGLE=0 doesn't overrides the original configuration flag set to TRUE. Same time MY_TOGGLE=1 works.

Steps to reproduce

Works with any boolean configuration field, used "Aggregate CSS files" for example:
- Create Environment variable "AGGREGAGE_CSS=0"
- Setup Key Configuration Override for Simple Configuration "system.performance" -> "css.preprocess"
- Check that the value is overridden

Proposed resolution

Check environment variable value more carefully and override configuration only on getenv() error (FALSE return value) or the variable is empty (empty string)