All commands that perform any kind of text assertion, perform white-space normalization on both, the input text and the assertion value. This is because browsers also normalize white-space when rendering text content in HTML.
White-space normalization in this context means that all white-space characters are replaced with a single blank space and the result is finally trimmed (white-space at the beginning and at the end are removed).
When reading and parsing the script files, white-space normalization is also performed for all command values that are known to treat with element text.
As a workaround, you might use one of the text matching strategies regexp or regexpi that interpret the assertion value as regular expression, e.g.
assertText id=targetElementId regexp:\s
In case it does not matter whether the displayed text is blank or empty, a text assertion with an empty value should do it.
Hope that helps.