Instead of using “Join Lines” you could do a regular expression replace-all operation:
Find-what box:  \R
Replace-with box:  make sure this box is EMPTY
Search mode: ☑ Regular expression
☑ Wrap around (maybe…depending upon what you want…)
“Join Lines” works on the active selection; if you want the replace operation to do likewise, then:  ☑ In selection
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