Restore support for 0-length grid lines.
Restore support for 0-length grid lines.

Resolves #3503

rachel-fenichel/blockly,mark-friedman/blockly,google/blockly,rachel-fenichel/blockly,google/blockly,mark-friedman/blockly,mark-friedman/blockly,google/blockly,google/blockly,mark-friedman/blockly,mark-friedman/blockly,google/blockly,rachel-fenichel/blockly,rachel-fenichel/blockly,rachel-fenichel/blockly,rachel-fenichel/blockly,rachel-fenichel/blockly,mark-friedman/blockly,google/blockly,google/blockly