Handle empty path on Windows

Author: stevapple

Sources/TSCBasic/Path.swift

@@ -685,6 +685,9 @@ private struct UNIXPath: Path {
 
     init(validatingAbsolutePath path: String) throws {
       #if os(Windows)
+        guard path != "" else {
+            throw PathValidationError.invalidAbsolutePath(path)
+        }
         let fsr: UnsafePointer<Int8> = path.fileSystemRepresentation
         defer { fsr.deallocate() }
 
@@ -707,6 +710,9 @@ private struct UNIXPath: Path {
 
     init(validatingRelativePath path: String) throws {
       #if os(Windows)
+        guard path != "" else {
+            self.init(normalizingRelativePath: path)
+        }
         let fsr: UnsafePointer<Int8> = path.fileSystemRepresentation
         defer { fsr.deallocate() }