`tooltip#sanitizeHtml()` missing `null` check

Created by: Klinton90

Operating system: Windows 10 Browser: Chrome 77

Test case: In update to 3.4.1 community added sanitizeHtml feature to tooltip.js. Code itself works fine, but it is missing null check before attempt to sanitize string/content/title, which breaks (at least in my project) existing code.

Proposed fix:

function sanitizeHtml(unsafeHtml, whiteList, sanitizeFn) {
    // if (unsafeHtml.length === 0) {
    if (!unsafeHtml || unsafeHtml.length === 0) {
      return unsafeHtml
    }
...
}