The non-surprising chaser vav in Leshono -- should we darshen it?

Summary: In Midrash Lekach Tov, a derasha on a chaser spelling of a word. But is it fit to make such a derasha, when this is the entirely expected spelling?

Post: In Shemot 11:7,

ז  וּלְכֹל בְּנֵי יִשְׂרָאֵל, לֹא יֶחֱרַץ-כֶּלֶב לְשֹׁנוֹ, לְמֵאִישׁ, וְעַד-בְּהֵמָה--לְמַעַן, תֵּדְעוּן, אֲשֶׁר יַפְלֶה יְהוָה, בֵּין מִצְרַיִם וּבֵין יִשְׂרָאֵל.

7 But against any of the children of Israel shall not a dog whet his tongue, against man or beast; that ye may know how that the LORD doth put a difference between the Egyptians and Israel.

Minchas Shai writes on this:
לֹא יֶחֱרַץ-כֶּלֶב לְשֹׁנוֹ -- in midrash Lekach Tov: "leshono is written chaser vav, that is to say that it will not harm them either with its teeth or its tongue." And even though all loshonos in the Torah are like it in terms of being spelled chaser, we saw fit to write it down, because it is the work of Heaven.

You can see this midrash Lekach Tov = Pesikta Zutrasa here. In terms of this post-Talmudic derasha, it is interesting. Presumably, since the vav is missing, one can read שנו at the end of the word, or perhaps the entire word as le-shino. But it calls to mind Ibn Ezra's lengthy objection to this sort of derasha, or darshening a word when it is written plene and the same word when it is written deficiently. In his view, there should at the least be something "askew" with the word which impels the derasha. (He goes further than that, actually, but that is for another post. In short, if a word is spelled sometimes malei and sometimes chaser, then that might simply indicate that there is no real regularity of spelling.)