fix: assets source become undefined in some scenario

[icelam]

Description

Related to issue #1, where it demonstrate the use of multiple html-webpack-plugin instances in a single webpack compiler.

In that scenario, the script assets was deleted early in the first beforeEmit hook, causing the build to throw:

ERROR in   TypeError: Cannot read property 'source' of undefined
  
  - HtmlInlineScriptPlugin.js:52 HtmlInlineScriptPlugin.processScriptTag
    [project-name]/[html-inline-script-webpack-plugin]/dist/HtmlInlineScriptPlugin.js:52:34
  
  - HtmlInlineScriptPlugin.js:67 
    [project-name]/[html-inline-script-webpack-plugin]/dist/HtmlInlineScriptPlugin.js:67:103
  
  - Array.map
  
  - HtmlInlineScriptPlugin.js:67 
    [project-name]/[html-inline-script-webpack-plugin]/dist/HtmlInlineScriptPlugin.js:67:69
  
  
  - new Promise
  
  
  - index.js:190 
    [project-name]/[html-webpack-plugin]/index.js:190:87

How has this been tested?

Have setup a minimal webpack which uses html-webpack-plugin instances in a single webpack compiler. Test the build is successful.

Types of changes

• New feature - feat
• Bug fix - fix
• Refactor - refactor
• Test cases - test
• Other(s):

Remarks

N/A