Clean up processing of input of string-to-document Automation type adapter

StringToDocModel (or TypeAdapterHelper after NXP-21528 refactoring) has curious code using with value.startsWith(".").

This code should be cleaned up because the actual intent seems to be to test for relative paths, which means that it needs to test value.equals(".") || value.equals("..") || value.startsWith("./") || value.startsWith("../"). In addition, unit tests for these case should be added, because for instance a value of ".foobar" shouldn't be resolved as a relative path (it should raise an exception).