Bug: getenv does not properly detect set-but-empty variable

[mjcarroll]

Discovered when writing some logic that checks for the presence of an environment variable, rather than the value.

You could in theory do something like

# In bash/zsh
export IGN_ENV_VAR=""

# and then in C++
ignition::common::env("IGN_ENV_VAR", foo)  # returns FALSE, but should be TRUE

This is completely valid (at least on Posix systems, unsupported on Windows)

Ref: https://github.com/ignitionrobotics/ign-common/blob/526c2aceb847dcdbfc2d25fc72457caaece03d3c/src/Util.cc#L328-L333

[mjcarroll]

Close by #97