Fix crash when passing empty strings to bytes.concat()

test/libsolidity/semanticTests/array/concat/bytes_concat_empty_strings.sol

@@ -0,0 +1,27 @@
+contract C {
+    function f() public returns (bytes memory) {
+        bytes memory b = "";
+        return bytes.concat(
+            bytes.concat(b),
+            bytes.concat(b, b),
+            bytes.concat("", b),
+            bytes.concat(b, "")
+        );
+    }
+
+    function g() public returns (bytes memory) {
+        return bytes.concat("", "abc", hex"", "abc", unicode"");
+    }
+
+    function h() public returns (bytes memory) {
+        bytes memory b = "";
+        return bytes.concat(b, "abc", b, "abc", b);
+    }
+}
+// ====
+// compileToEwasm: also
+// compileViaYul: also
+// ----
+// f() -> 0x20, 0
+// g() -> 0x20, 6, "abcabc"
+// h() -> 0x20, 6, "abcabc"

test/libsolidity/syntaxTests/array/concat/bytes_concat_wrong_type_empty_string_literal.sol

@@ -0,0 +1,6 @@
+contract C {
+    function f() public pure {
+        bytes.concat(hex"", unicode"", "");
+    }
+}
+// ----