Explicitly handle empty paths

Author: stevapple

Sources/TSCBasic/Path.swift

@@ -692,6 +692,12 @@ private struct UNIXPath: Path {
 
     init(validatingAbsolutePath path: String) throws {
       #if os(Windows)
+        // Explicitly handle the empty path, since retrieving
+        // `fileSystemRepresentation` of it is illegal.
+        guard !path.isEmpty else {
+            throw PathValidationError.invalidAbsolutePath(path)
+        }
+
         let fsr: UnsafePointer<Int8> = path.fileSystemRepresentation
         defer { fsr.deallocate() }
 
@@ -714,6 +720,12 @@ private struct UNIXPath: Path {
 
     init(validatingRelativePath path: String) throws {
       #if os(Windows)
+        // Explicitly handle the empty path, since retrieving
+        // `fileSystemRepresentation` of it is illegal.
+        guard !path.isEmpty else {
+            throw PathValidationError.invalidRelativePath(path)
+        }
+
         let fsr: UnsafePointer<Int8> = path.fileSystemRepresentation
         defer { fsr.deallocate() }