Yes you can definitely practice guitar quietly and without an amp. Using an amp, or playing a guitar with full dynamics/loudness has it's place as well. But if you're in a jam and can't make too much noise, it's ok to practice softly. Practicing softly can help you have more control of your dynamics, and practicing without an amp can still get you better at the actual technique of playing, even if you can't hear how your playing sounds in it's "full glory."