Check if EditorConfig file exist for Ktlint in KotlinGradleExtension

[Goooler]

I missed this in #1788.

[Goooler]

This had been finished in the Maven plugin at

spotless/plugin-maven/src/main/java/com/diffplug/spotless/maven/kotlin/Ktlint.java

Line 45 in 13a36fc

configPath = ThrowingEx.get(() -> FileSignature.signAsList(stepConfig.getFileLocator().locateFile(editorConfigPath)));

right?

[nedtwigg]

Yep, signAsList throws an error message on its own, but writing it out explicitly as you did in this PR is good too.

[nedtwigg]

Released in plugin-gradle 6.23.0 and plugin-maven 2.41.0.