refactor(attr): treat empty string as no binding

[bigopon]

📖 Description

Currently, custom attributes with single value binding have a weirdness in their usage related to empty string, or no value usage. Consider the following template:

<div my-attr>

There is ambiguity between no-value and empty string as value for the primary bindable.
Current behavior is we are assuming attribute wants to have an empty string but this is not ideal.

BREAKING CHANGE:

This PR changes the default behavior when there's no value (<div attr>), or an empty string value (<div attr="">) for a custom attribute usage, so that now there will be no binding created for the primary bindable. Applications that truely want to bind to an empty string can always use empty string expression like the following example:

<div my-attr.bind="">
<div my-attr.bind="``">

🎫 Issues

resolves #1911

cc @Sayan751 @fkleuver

[codecov]

Codecov Report

Attention: Patch coverage is 92.30769% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 88.62%. Comparing base (49e327e) to head (c469d57).
Report is 2 commits behind head on master.

Files	Patch %	Lines
packages/runtime-html/src/templating/styles.ts	66.66%	1 Missing ⚠️
packages/runtime/src/binding/ast.visitor.ts	0.00%	1 Missing ⚠️
packages/state/src/state-templating.ts	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1930      +/-   ##
==========================================
+ Coverage   88.58%   88.62%   +0.04%     
==========================================
  Files         262      262              
  Lines       22870    22891      +21     
  Branches     5282     5288       +6     
==========================================
+ Hits        20260    20288      +28     
+ Misses       2610     2603       -7     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.