from nltk.tokenize import sent_tokenize

def split_into_sentences(input_text):
    """
    Splits input text into sentences by newlines.

    Args:
        input_text: The input text as a string.

    Returns:
        A list of sentences. Returns an empty list if input is not valid.
    """
    if not isinstance(input_text, str):
        return []

    paragraphs = input_text.splitlines()
    sentences = []
    for paragraph in paragraphs:
        paragraph = paragraph.strip()
        if paragraph:
            sentences.extend(sent_tokenize(paragraph))
    return sentences