feat: use `undefined` as fallback for `string` & `number` args with no values

[kricsleo]

resolves #142

This PR changes the fallback for string & number args from "" to undefined.

This change enables differentiation between user-specified values, such as: test --config vs test --config ""

Example:

  args: {
    name: {
      type: 'string',
    },
    age: {
      type: 'number',
    },
  },

Previously:

test --name => ctx.args.name === "" test --name "" => ctx.args.name === "" test --age => throws

Now:

test --name => ctx.args.name === undefined & ("name" in ctx.args) === true test --name "" => ctx.args.name === ""

test --age => ctx.args.age === undefined & ("age" in ctx.args) === true

[codecov[bot]]

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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #195   +/-   ##
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  Coverage        ?   94.10%           
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  Files           ?        7           
  Lines           ?      509           
  Branches        ?      166           
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  Hits            ?      479           
  Misses          ?       30           
  Partials        ?        0           

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