Do you put garlic around your windows every night, just in case there are vampires lurking? Do you put milk out for the potential fairies at the bottom of your garden as well? There are millions of other hypothetical mythical beings we dismiss although we can't absolutely "prove the negative" of their non-existence with logical certitude (apart from the self-contradictions in many mythological concepts).
If we should have expected to see evidence for something by now but have not, then we should conclude it probably doesn't exist. If possible evidence is rare but may occur in the future, we should consider it possible but not yet demonstrated. If there is no evidence and it will not be possible to find evidence for it in the future than it should be considered meaningless and not believable. So yes, lack of evidence can lead you to a robust conclusion.
As an empiricist concepts are valuable to me if they help make sense of the world, and meaningless or unsupported concepts are damaging as they take up my limited brainpower and time which could be used more productively. In real life we never have 100% (dis)proof of anything, but that should not stop us removing generally useless or misleading ideas from our mental toolkit.
The distinction between gnosticism and agnosticism is a false dichotomy as complete knowledge or certainty doesn't exist outside axiomatic systems and there is almost always some evidence for or against a belief. The appropriate judgment is whether you have sufficient confidence in your conclusions that they inform your everyday decisions. For example I have no more belief in supernatural entities than in the possibility that a black hole is hiding in my kettle and will swallow me up when I next go to make a cup of tea, and act accordingly. In practice sufficiently unlikely possibilities must be ignored.
Of course I could change my mind given new evidence. If the stars in the sky rearranged themselves to spell out "Ragnarok is coming next Thursday" in Old Norse I would reevaluate my beliefs about Odin (after checking I was not hallucinating). However that possibility does not mean I am a semi-theistic agnostic with the currently available evidence.