Given a sentence of space-separated words, return the most frequently occurring word. Comparison is case-insensitive and the returned word must be lowercase. If several words tie for the highest count, return the one that appears first in the sentence. For a null or blank sentence, return an empty string. Assume words contain no punctuation.
Examples:
Input: sentence = 'the cat the dog the bird'
Output: 'the'
Explanation: 'the' appears 3 times, more than any other word
Input: sentence = 'apple banana apple cherry banana apple'
Output: 'apple'
Explanation: 'apple' appears 3 times
Input: sentence = 'one two three'
Output: 'one'
Explanation: All words appear once, so return the first word
Input: sentence = 'Hello hello HELLO world'
Output: 'hello'
Explanation: Case-insensitive count makes 'hello' appear 3 times
Input: sentence = ''
Output: ''
Explanation: A blank sentence returns an empty string
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