Fix webpack config to account for `NO_SOURCEMAPS=false`

What does this MR do?

A recent update to the GDK adds the NO_SOURCEMAPS environment variable to the Procfile based on config in gdk.yml. The NO_SOURCEMAPS environment variable will now be provided regardless of whether it was enabled or disabled, but the webpack config was relying on the mere existence of this variable to disable sourcemaps. So NO_SOURCEMAPS=false was treated the same as NO_SOURCEMAPS=true. This fixes this behavior for multiple environment variables within webpack.config.js to explicitly handle cases where "false" is provided.

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Does this MR meet the acceptance criteria?

Conformity

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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
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Security

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