Handle empty files and truncated YAML front matter

[mbovel]

Scaladoc crashes when a static file is empty:

[error] Caused by: java.util.NoSuchElementException: head of empty list
[error] 	at scala.collection.immutable.Nil$.head(List.scala:662)
[error] 	at scala.collection.immutable.Nil$.head(List.scala:661)
[error] 	at dotty.tools.scaladoc.site.common$package$.loadTemplateFile(common.scala:66)
[error] 	at dotty.tools.scaladoc.site.StaticSiteLoader.loadRecursively(StaticSiteLoader.scala:191)
[error] 	at dotty.tools.scaladoc.site.StaticSiteLoader.loadRecursively$$anonfun$1(StaticSiteLoader.scala:188)

See sbt/sbt#7256 for more context.

This PR solves this issue, and also handles unended YAML front matter.