Don't insert an extra break inside empty multiline strings.

Sources/SwiftFormatPrettyPrint/TokenStreamCreator.swift

@@ -2287,7 +2287,9 @@ fileprivate final class TokenStreamCreator: SyntaxVisitor {
       // Looks up the correct break kind based on prior context.
       let breakKind = pendingMultilineStringBreakKinds[node, default: .same]
       after(node.openQuote, tokens: .break(breakKind, size: 0, newlines: .hard(count: 1)))
-      before(node.closeQuote, tokens: .break(breakKind, newlines: .hard(count: 1)))
+      if !node.segments.isEmpty {
+        before(node.closeQuote, tokens: .break(breakKind, newlines: .hard(count: 1)))
+      }
     }
     return .visitChildren
   }

Tests/SwiftFormatPrettyPrintTests/StringTests.swift

@@ -350,6 +350,7 @@ final class StringTests: PrettyPrintTestCase {
 
     assertPrettyPrintEqual(input: input, expected: expected, linelength: 20)
   }
+
   func testLeadingMultilineStringsInOtherExpressions() {
     // The stacked indentation behavior needs to drill down into different node types to find the
     // leftmost multiline string literal. This makes sure that we cover various cases.
@@ -417,4 +418,44 @@ final class StringTests: PrettyPrintTestCase {
       """#
     assertPrettyPrintEqual(input: input, expected: input + "\n", linelength: 100)
   }
+
+  func testEmptyMultilineStrings() {
+    let input =
+      ##"""
+      let x = """
+        """
+      let y =
+        """
+        """
+      let x = #"""
+        """#
+      let y =
+        #"""
+        """#
+      """##
+
+    assertPrettyPrintEqual(input: input, expected: input + "\n", linelength: 20)
+  }
+
+  func testOnlyBlankLinesMultilineStrings() {
+    let input =
+      ##"""
+      let x = """
+
+        """
+      let y =
+        """
+
+        """
+      let x = #"""
+
+        """#
+      let y =
+        #"""
+
+        """#
+      """##
+
+    assertPrettyPrintEqual(input: input, expected: input + "\n", linelength: 20)
+  }
 }