Yes. The halacha seems to be that if you hire someone to guard hefker items, and the 'employee' knows that it is hefker and is not laboring under the impression that it is yours, then you do not owe the 'employee' anything at all.
This can be found in Shulchan Aruch 336:3:
שכרו לשמור לו דבר של הפקר יכול לומר לו טול מה שעשית בשכרך לפי שעדיין לא זכה בו ב"ה וילך הוא ויזכה בו:
If he hired him to guard a hefker item, then he can say to him, "Take what you labored with as your salary", since the employer has not yet acquired it, and he [the employee] goes and acquires it.
Sma (ad loc. 10) clarifies two important points:
עצה טובה הוא זה להשומר שילך ויקדים נפשו לזכות בו מן ההפקר דהיינו בהגבהה או משיכה דאם יקדימוהו אחר לזכות בו יהי' קרח מכאן ומכאן כיון דידע דשל הפקר הוא הרי הוא כאלו שכרו לעשות בשדה של חבירו וידע בו הפועל
He is giving good advice to the guard that he should go first and acquire the items from hefker by picking them up or dragging them, because if someone else gets there first the worker will lose out on both sides [he won't get the hefker items and also has no claim on the 'employer'] since he knew that it was a hefker item, it is equivalent to the case where he employed him to work on his friend's field and the worker knew this [where the halacha is that the 'employer' is not liable].
So, from Sma we see that:
- Yes, the 'employer' is not liable in this case where he gains nothing from the 'employee's' work, so long as,
- The 'employee' is aware of the circumstances under which he is working.
You asked whether this beraita (as interpreted by R. Nachman) is the source of this halacha. Be'er HaGolah (ad loc. 10) confirms that it is.
Your final question, why does the gemara talk about telling the 'employee' to help himself to the hefker rather than stating straight out the the 'emplyer' is not liable, is asked by Tosafot (Bava Metzia 118a s.v. kan beshel chaveiro) who do not provide an answer:
תימה למאי דס"ד השתא אמאי צריך לומר לו טול מה שעשית בשכרך יאמר לא אתן לך כלום שלא עשית לי כלום:
It is astounding. According to what we think now, why does he need to say, "Take what you did as your wage"? He should say to him, "I will not give you anything, because you did nothing for me."
(Note that Tosafot ask the question at an earlier stage of the gemara's discussion, where it is attempting to draw a distinction between the employer's own field and his friend's field. However, I believe, the underlying logic is identical.)