skip semicolon in stmtlist expr parsing (#24855)

Previously it would try to parse the semicolon as its own statement and
produce an `nkEmpty` node

Also more than 1 semicolon in an expression list i.e. `(a;; b)` gives an
"expression expected" error in `semiStmtList` when multiple semicolons
are allowed in normal statements, this could be fixed by changing the
`if tok.kind == tokSemicolon` check to a `while` but it does not match
the grammar so not done here.

(cherry picked from commit 918f972369)

compiler/parser.nim

@@ -699,10 +699,12 @@ proc parsePar(p: var Parser): PNode =
       asgn.add b
       result.add(asgn)
       if p.tok.tokType == tkSemiColon:
+        getTok(p)
         semiStmtList(p, result)
     elif p.tok.tokType == tkSemiColon:
       # stmt context:
       result.add(a)
+      getTok(p)
       semiStmtList(p, result)
     else:
       a = colonOrEquals(p, a)

tests/parser/tstmtlistexprempty.nim

@@ -0,0 +1,23 @@
+discard """
+  nimout: '''
+StmtList
+  ReturnStmt
+    StmtListExpr
+      Call
+        DotExpr
+          Ident "x"
+          Ident "add"
+        StrLit "123"
+      Call
+        DotExpr
+          Ident "x"
+          Ident "add"
+        StrLit "123"
+      Ident "x"
+'''
+"""
+
+import std/macros
+
+dumpTree:
+  return (x.add("123"); x.add("123"); x)