feat!: set default if var is set but empty

[caarlos0]

BREAKING CHANGE: before this, env would set the default value for a variable only if the variable was never set, and would do nothing if it was set to an empty value. After this, it will set the default value if empty as well.

closes #245

[codecov]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage 👍

Coverage data is based on head (402cae8) compared to base (da848aa).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #248   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines          356       358    +2     
=========================================
+ Hits           356       358    +2     

Impacted Files	Coverage Δ
env.go	100.00% <100.00%> (ø)

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[arvindh123]

It would great of there is some thing opposite to notEmpty tag, like allowEmpty tag. ?

Example :

type user struct {
     subscribe `env:"SUBSCRIBE,allowEmpty" envDefault:"yes"`
}

[caarlos0]

@arvindh123 tbh I think that would be confusing... rather keep it with fewer different behaviors...

[arvindh123]

@arvindh123 tbh I think that would be confusing... rather keep it with fewer different behaviors...
@caarlos0

It will be useful and It gives some more flexibility.
In similar kind of library , Viper there is option to treat empty environment variables as set
Quick reference link : https://github.com/spf13/viper#working-with-environment-variables

[dborovcanin]

Hello, @caarlos0. We plan on using this lib on the Mainflux project here. I agree with Arvindh's comment since it would provide some extra flexibility, but for our needs, this PR is sufficient as is. Any idea when it will be merged?

[caarlos0]

I still havent made my mind on this...

[drasko]

@caarlos0 this one is blocking Mainflux development, so if it can't be merged by the end of this week we'll have to fork on Monday. I think it would be a pity to fragment the dev, so I hope there will be no need for fork.

[caarlos0]

Okay, I made up my mind.