Fix overwriting first action of a noneBlock when overlaying layers.

[kareltucek]

For example config and reproduction steps see: #1419

Example:

• Assume:
  • layer 1: qwertyuiop
  • layer 2: Q__Q_Q____
  • where _ stands for none action
• now overlayLayer 2 onto 1.
  • expected: QweQtQuiop
  • actual: Q_eQtQ_iop

Changelog:

• fix issue where, when using overlayLayer, the first empty action after any nonempty action would be written through. (When in a sequence of at least two empty actions, to be precise.)

Closes #1419