Add tests for ABI coding empty strings and string literals

Include the case of revert("")

test/libsolidity/semanticTests/strings/empty_string.sol

@@ -0,0 +1,9 @@
+contract C {
+    function f() public pure returns (string memory) {
+        return "";
+    }
+}
+// ====
+// compileViaYul: also
+// ----
+// f() -> 0x20, 0