fix: don't eliminate StringBuilder if no String arg present

jadx-core/src/main/java/jadx/core/dex/visitors/SimplifyVisitor.java

@@ -332,6 +332,20 @@ public class SimplifyVisitor extends AbstractVisitor {
 				}
 				args.add(arg);
 			}
+
+			boolean stringArgFound = false;
+			for (InsnArg arg : args) {
+				if (arg.getType().equals(ArgType.STRING)) {
+					stringArgFound = true;
+					break;
+				}
+			}
+			if (!stringArgFound) {
+				// TODO: convert one arg to string using `String.valueOf()`
+				return null;
+			}
+
+			// all check passed
 			removeStringBuilderInsns(mth, toStrInsn, chain);
 
 			InsnNode concatInsn = new InsnNode(InsnType.STR_CONCAT, args);

jadx-core/src/test/java/jadx/tests/integration/others/TestStringBuilderElimination4Neg.java

+package jadx.tests.integration.others;
+
+import org.junit.jupiter.api.Test;
+
+import jadx.core.dex.nodes.ClassNode;
+import jadx.tests.api.IntegrationTest;
+
+import static org.hamcrest.CoreMatchers.containsString;
+import static org.hamcrest.MatcherAssert.assertThat;
+
+public class TestStringBuilderElimination4Neg extends IntegrationTest {
+
+	public static class TestCls<K, V> {
+		private K k;
+		private V v;
+
+		public String test() {
+			StringBuilder sb = new StringBuilder();
+			sb.append(k);
+			sb.append('=');
+			sb.append(v);
+			return sb.toString();
+		}
+	}
+
+	@Test
+	public void test() {
+		ClassNode cls = getClassNode(TestCls.class);
+		String code = cls.getCode().toString();
+
+		assertThat(code, containsString("sb.append('=');"));
+	}
+}