by.buttonText('') does not return empty texted buttons as protractor locator

I have to test an application that has some buttons with no text, only graphics (these are < and > buttons on a datepicker, for instance, not in my control).

If I try to find these by:

elm = element.all(by.buttonText(''));
backButton = elm.get(1);

I get no immediate error, but if I say backButton.click() I get:

Error: Index out of bound. Trying to access element at index: 1, but there are only 0 elements that match locator by.buttonText("").

On the other hand if I say

element.all(by.tagName("BUTTON")).filter(function (elem) {
    return elem.getText().then(function (text) {

        //find the empties
        return !(text);
        //
    });
}).then(function (EmptyButtons) {

    EmptyButtons[1].click()

I get a fun error suggesting I found one that I clicked did not pop up yet:

JavascriptError: Element is not currently visible and may not be manipulated (status: 11)

but that is on me to figure out. If I track the text as it comes through the filter, I find there are multiple text-less buttons placed in the array.

I am sorry I do not yet have the skills to generate and test a small example, but I hope perhaps some helpful reader does. Anyway, I suppose it is possible I am missing something subtle about promise resolution or that the desired behavior is you cannot find a textless button by looking for empty text. I have a workaround, too (actually multiple ones - see my comment below). But I would welcome some clarification here if someone can give it. Best, Jeremy Kahan p.s. I do not see this as a duplicate of "How to click a button with no text in protractor" inasmuch as that wrestles with distinguishing textless buttons, and my question is not how to find and distinguish them, but why searching for text '' does not find them. [My code even suggests how that could have been or could still be implemented, I think.]