run_before_script: Fix newlines (#960)

Follow up to #958, #959

Author: tony

CHANGES

@@ -19,6 +19,10 @@ $ pipx install --suffix=@next 'tmuxp' --pip-args '\--pre' --force
 
 <!-- Maintainers, insert changes / features for the next release here -->
 
+### Bug fixes
+
+- `run_before_script()`: Additional output capturing improvements (#960)
+
 ## tmuxp 1.52.1 (2025-02-02)
 
 ### Bug fixes

src/tmuxp/util.py

@@ -65,13 +65,13 @@ def run_before_script(
         # Read one line from stderr, if available
         line_err = proc.stderr.readline() if proc.stderr else ""
 
-        if line_out:
+        if line_out and line_out.strip():
             out_buffer.append(line_out)
             if is_out_tty:
                 sys.stdout.write(line_out)
                 sys.stdout.flush()
 
-        if line_err:
+        if line_err and line_err.strip():
             err_buffer.append(line_err)
             if is_err_tty:
                 sys.stderr.write(line_err)