It may be useful to conceptualize the transistor as a current source instead, but the end result looks a lot like a divider… Six of one, half dozen of the other…
An empty space where a resistor isn’t may indeed be thought an “infinite” resistance for many practical purposes. If, as indicated previously, you’re only able to achieve a 0.3v change across that, it’s an indication that your current source (transistor) isn’t doing much at all.
A simple reading of the datasheet indicates that phototransistor should pass 0.1uA (at most) in the dark and at least 100uA under modest light, with a minimum 0.4v drop across the device to be expected.
Stated differently, you shouldn’t be having much trouble getting a signal that varies between 0 and 4.5v or so across resistors as low as 50K.
Have you double-checked the polarity of the transistor in the circuit?