fix: empty string passed to Base64.decode produces blank b64: comment

Author: nklapste

src/main/java/jadx/plugins/stringdecoder/B64Detector.java

@@ -166,6 +166,9 @@ public static B64Result decodeForced(String str, int maxCommentLength) {
 		for (int i = 0; i < ALL_DECODERS.length; i++) {
 			try {
 				byte[] bytes = ALL_DECODERS[i].decode(str);
+				if (bytes.length == 0) {
+					return null;
+				}
 				CharsetDecoder utf8 = StandardCharsets.UTF_8.newDecoder()
 						.onMalformedInput(CodingErrorAction.REPLACE)
 						.onUnmappableCharacter(CodingErrorAction.REPLACE);

src/test/java/jadx/plugins/stringdecoder/JadxStringDecoderPluginTest.java

@@ -29,6 +29,15 @@ public void notB64Test() throws Exception {
 		assertThat(code).doesNotContain("b64:");
 	}
 
+	@Test
+	public void emptyStringB64ArgTest() throws Exception {
+		// Empty string passed to Base64.decode — decodes to zero bytes, no comment should appear.
+		// Regression: decodeForced("") previously produced "// b64: " (empty decoded value).
+		String code = decompileSmali("b64/empty_string_b64_arg.smali");
+		System.out.println(code);
+		assertThat(code).doesNotContain("b64:");
+	}
+
 	@Test
 	public void looksLikeB64Test() throws Exception {
 		// "AQIDBAUG" matches the Base64 charset but decodes to binary [1,2,3,4,5,6] — not printable UTF-8, no comment expected

src/test/resources/samples/b64/empty_string_b64_arg.smali

@@ -0,0 +1,14 @@
+.class public Lb64/EmptyStringB64Arg;
+.super Ljava/lang/Object;
+
+# Empty string passed to Base64.decode — real-world pattern where "" is loaded early
+# and later used as the input to a decode call (e.g. as a default/fallback value).
+.method public static process()V
+    .registers 3
+
+    const-string v0, ""
+    const/4 v1, 0x0
+    invoke-static {v0, v1}, Landroid/util/Base64;->decode(Ljava/lang/String;I)[B
+    move-result-object v0
+    return-void
+.end method