Adapt FenceStep for the reality that we haven't shipped linting yet.

Author: nedtwigg

lib/src/main/java/com/diffplug/spotless/generic/FenceStep.java

@@ -18,6 +18,7 @@
 import java.io.File;
 import java.io.Serializable;
 import java.nio.charset.StandardCharsets;
+import java.nio.file.Path;
 import java.util.ArrayList;
 import java.util.List;
 import java.util.Objects;
@@ -176,6 +177,7 @@ protected Formatter buildFormatter() {
 					.encoding(StandardCharsets.UTF_8) // can be any UTF, doesn't matter
 					.lineEndingsPolicy(LineEnding.UNIX.createPolicy()) // just internal, won't conflict with user
 					.steps(steps)
+					.rootDir(Path.of("")) // TODO: error messages will be suboptimal for now, but it will get fixed when we ship linting
 					.build();
 		}