skip undefined attributes

[Marshevskyy]

in order to support the same functionality (skip attributes when passing undefined) as you had with xmlbuilder v1. Added small fix.

one more reason why I added this fix is:
when we using TypeScript with optional values as function arguments we do not want to set null as a default value, and we want to use original argument value, which is undefined by default (if the case if we do not pass value)

instead of:

  constructor(closed?: boolean) {
    this.node = parentNode
      .element('RoomType')
      .attribute('id', exRoomId);

    if (closed != null) {
      this.node.attribute('closed', closed);
    } 

we want to use it like

  constructor(closed?: boolean) {
    this.node = parentNode
      .element('RoomType')
      .attribute('id', exRoomId);

    this.node.attribute('closed', closed); // in case if closed undeffined I expect to skip this att

[codecov-io]

Codecov Report

Merging #5 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master       #5   +/-   ##
=======================================
  Coverage   99.46%   99.46%           
=======================================
  Files          13       13           
  Lines        1669     1669           
  Branches      482      482           
=======================================
  Hits         1660     1660           
  Misses          9        9

Impacted Files	Coverage Δ
src/builder/XMLBuilderImpl.ts	100% <100%> (ø)	⬆️

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