Search and Replace
Under Edit › Find & Replace (Ctrl + f) you can search through your whole document for a particular string of text and also replace it with some other string.
I myself very often use the search tool for a quite different purpose: I ‘mark’ passages in need of further editing with the optically salient ###, allowing me to easily locate these passages at a later date.
You can even replace a paragraph style with another style (select More Options). This is especially useful to rid yourself of superfluous styles.
Finally you can remove all empty paragraphs in one go: Under More Options choose Regular expressions and write ^$ in the search field, then press Replace all.