Add empty input tests

Author: CodaFi

Sources/FileCheck/FileCheck.swift

@@ -105,9 +105,13 @@ public func fileCheckOutput(of FD : FileCheckFD = .stdout, withPrefixes prefixes
   }
 
   let input = overrideFDAndCollectOutput(file: FD, of: block)
-  if (input.isEmpty && !options.contains(.allowEmptyInput)) {
-    print("FileCheck error: input from file descriptor \(FD) is empty.\n")
-    return false
+  if input.isEmpty {
+    guard options.contains(.allowEmptyInput) else {
+      print("FileCheck error: input from file descriptor \(FD) is empty.\n")
+      return false
+    }
+    
+    return true
   }
 
   guard let contents = try? String(contentsOfFile: file, encoding: .utf8) else {

Tests/FileCheckTests/EmptySpec.swift

@@ -0,0 +1,19 @@
+import FileCheck
+import XCTest
+import Foundation
+
+class EmptySpec : XCTestCase {
+//  func testEmptyError() {
+//    XCTAssert(fileCheckOutput(of: .stdout, withPrefixes: ["EMPTY-ERR"]) {
+//      // EMPTY-ERR: FileCheck error: input from file descriptor stdout is empty.
+//      XCTAssertFalse(fileCheckOutput(options: [.disableColors]) {})
+//    })
+//  }
+
+  func testAllowEmpty() {
+    XCTAssert(fileCheckOutput(of: .stdout, withPrefixes: ["ALLOW-EMPTY-ERR"], options: [.allowEmptyInput]) {
+      // ALLOW-EMPTY-ERR-NOT: FileCheck error: input from file descriptor stdout is empty.
+      XCTAssert(fileCheckOutput(options: [.allowEmptyInput]) {})
+    })
+  }
+}