Fixed bug: empty text lines had too large height

Summary:
If one of the lines in a text item contained only whitespace,
the height would be too large.

Test Plan:
Insert a text item where the first line
has non-whitespace text, the second line has whitespace text
only, and the third line has non-whitespace text. Observe that
the text item is rendered correctly.

Reviewers: davidb

Reviewed By: davidb

Differential Revision: https://phab.met.no/D54