All Upper II
Check if a given string has all symbols in upper case. If the string is empty or doesn't have any letter in it - function should return False.
Input: A string (str).
Output: A logic value (bool).
Examples:
assert is_all_upper("ALL UPPER") == True
assert is_all_upper("all lower") == False
assert is_all_upper("mixed UPPER and lower") == False
assert is_all_upper("") == False
Precondition: a-z, A-Z, 1-9 and spaces.
