nothing to say vs say nothing??

noting to say - 無話可說 ( 語氣是帶着無奈的 )。

e.g. I have nothing to say if she insisted on blaming me who is the one to break her heart.
如果她堅持是我令她的心破碎，我無話可說。

say nothing - 沒有說任何話 ( 直接或帶有答覆的意思 ) 。

e.g. I say ( said ) nothing to that stranger.
我沒有對那陌生人說任何話。

Nothing to say means
無嘢可以講　（唔係唔想講，係無嘢可以講）

say nothing means
無講到嘢　　（可能有嘢可以講，亦可能無嘢可以講，　但最重要係無講到嘢）

I have nothing to say = 我無野好講/無話可說;
I say (said) nothing = 我冇講過/冇意見;

i have nothing to say means 我沒什麼可說.
i say nothing means 我沒說什麼.