All Upper I All Upper I
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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 True.

Input: A string.

Output: a boolean.


is_all_upper('ALL UPPER') == True
is_all_upper('all lower') == False
is_all_upper('mixed UPPER and lower') == False
is_all_upper('') == True
is_all_upper('444') == True
is_all_upper('55 55 5') == True

Precondition: a-z, A-Z, 1-9 and spaces