HelmValuesJsonSchemaProvider should be disabled when virtual file parent is null

[fstaudt]

Special case of null parent for virtual file should be handled in HelmValuesJsonSchemaProvider.
In such case, isAvailable should return false.

java.lang.NullPointerException: Cannot invoke "com.intellij.openapi.vfs.VirtualFile.getName()" because the return value of "com.intellij.openapi.vfs.VirtualFile.getParent()" is null
	at io.github.fstaudt.helm.idea.providers.HelmValuesJsonSchemaProvider.isAvailable(HelmValuesJsonSchemaProvider.kt:16)
	at com.jetbrains.jsonSchema.impl.JsonSchemaServiceImpl.isProviderAvailable(JsonSchemaServiceImpl.java:430)