3, ×, ÷, = displays NaN instead of 3

[a-n-d-r-3-w]

It seems that when the user presses two operators in succession, the code assumes the active operand is zero, and it inserts this zero into the calculator in between the two operators, even though the user did not explicitly press zero.

Related use cases:

• 6, ×, ÷, 2, = displays 0 instead of 3
• 6, ÷, × displays Infinity instead of 6
• 6, ÷, ×, = displays NaN instead of 6
• 5, ×, -, 6 displays -6 instead of -1
• 5, ÷, - displays "Infinity" instead of 5
• 8.9, ×, ÷ displays 0.3 instead of 8.9
• 777, ×, × displays 0 instead of 777
• 777, ÷, ÷ displays Infinity instead of 777