fix: Throw clear error when DSN is empty

Previously an empty or whitespace-only DSN string would fall through
to the URI constructor, producing a confusing error message. Now the
Dsn constructor checks for empty strings after trimming and throws
an IllegalArgumentException with a clear message.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: adinauer

sentry/src/main/java/io/sentry/Dsn.java

@@ -51,6 +51,9 @@ URI getSentryUri() {
   Dsn(@Nullable String dsn) throws IllegalArgumentException {
     try {
       final String dsnString = Objects.requireNonNull(dsn, "The DSN is required.").trim();
+      if (dsnString.isEmpty()) {
+        throw new IllegalArgumentException("The DSN is empty.");
+      }
       final URI uri = new URI(dsnString).normalize();
       final String scheme = uri.getScheme();
       if (!("http".equalsIgnoreCase(scheme) || "https".equalsIgnoreCase(scheme))) {

sentry/src/test/java/io/sentry/DsnTest.kt

@@ -95,6 +95,24 @@ class DsnTest {
     assertEquals("https://host/api/id", dsn.sentryUri.toURL().toString())
   }
 
+  @Test
+  fun `when dsn is empty, throws exception`() {
+    val ex = assertFailsWith<IllegalArgumentException> { Dsn("") }
+    assertEquals(
+      "java.lang.IllegalArgumentException: The DSN is empty.",
+      ex.message,
+    )
+  }
+
+  @Test
+  fun `when dsn is only whitespace, throws exception`() {
+    val ex = assertFailsWith<IllegalArgumentException> { Dsn("   ") }
+    assertEquals(
+      "java.lang.IllegalArgumentException: The DSN is empty.",
+      ex.message,
+    )
+  }
+
   @Test
   fun `non http protocols are not accepted`() {
     assertFailsWith<IllegalArgumentException> { Dsn("ftp://publicKey:secretKey@host/path/id") }