Is it obligatory to perform ghusl after the emission of a thick, scanty white fluid resembling semen, with no trace of semen present before pressing the penis?

You were not required to do what you did, namely, inspecting and squeezing the penis, because it causes obsessive doubts and may harm one's health. Most likely, the discharge is a wet, impure discharge that necessitates ablution (wudu') but not a ritual bath (ghusl). Even if it were semen, its discharge without gushing and pleasure does not necessitate a ritual bath; rather, ablution suffices. Your prayer is valid.