feat: better "obtained empty" help message (#502)

* feat: better "obtained empty" help message

Previously, when one used `assertNoDiff` and obtained string was empty there was no clue what does `expected` looks like. Because of that, sometimes it wasn't clear which comparison fails.
Now, in the case when `obtained` is empty, `expected` is being printed.

munit/shared/src/main/scala/munit/internal/difflib/Diffs.scala

@@ -39,7 +39,11 @@ object Diffs {
       printObtainedAsStripMargin: Boolean
   )(implicit loc: Location): Boolean = {
     if (obtained.isEmpty && !expected.isEmpty) {
-      handler.handle("Obtained empty output!", obtained, expected, loc)
+      val msg =
+        s"""|Obtained empty output!
+            |=> Expected:
+            |$expected""".stripMargin
+      handler.handle(msg, obtained, expected, loc)
     }
     val diff = new Diff(obtained, expected)
     if (diff.isEmpty) true

tests/shared/src/test/scala/munit/ComparisonFailExceptionSuite.scala

@@ -54,4 +54,21 @@ class ComparisonFailExceptionSuite extends BaseSuite {
     )
   }
 
+  test("assert-no-diff-obtained-empty") {
+    val e = intercept[ComparisonFailException] {
+      assertNoDiff("", "Lorem ipsum")
+    }
+    assertNoDiff(
+      e.getMessage(),
+      """|ComparisonFailExceptionSuite.scala:59
+         |58:    val e = intercept[ComparisonFailException] {
+         |59:      assertNoDiff("", "Lorem ipsum")
+         |60:    }
+         |Obtained empty output!
+         |=> Expected:
+         |Lorem ipsum
+         |""".stripMargin
+    )
+  }
+
 }