When the Wire.requestFrom() function returns, the I2C transaction has completely finished and the received data is waiting in a buffer in the Wire library. There is no need to wait for something.
In the file "FaBoHumidity-HTS221-Library/src/FaBoHumidity_HTS221.cpp" in the function "readI2c()", the line "while(!Wire.available());" after the Wire.requestFrom() can be removed.