Count the number of times a substring occurs inside a given text. Overlapping matches must be counted. For example, the substring "aa" appears three times in "aaaa". Return 0 when either argument is null, when the substring is empty, or when the substring is longer than the text.
Examples:
Input: text = 'hello world', sub = 'l'
Output: 3
Explanation: The character l appears at positions 2, 3, and 9
Input: text = 'aaaa', sub = 'aa'
Output: 3
Explanation: Overlapping matches at positions 0, 1, and 2
Input: text = 'abc', sub = 'd'
Output: 0
Explanation: The substring is not present in the text
Input: text = 'abc', sub = ''
Output: 0
Explanation: An empty substring returns zero by definition
Input: text = null, sub = 'a'
Output: 0
Explanation: Null text returns zero
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