Music: fix for infinite loop when loop field cleared

[breville]

When the "by" field was cleared, an infinite loop would occur, due to the generated code including something like this:

for (i = 1; i <= 3; i += 0)

The issue is subtle, but it turns out that Blockly.JavaScript.valueToCode was returning '0', and while '0' == false might be true, '0' || '1' will return '0'.

The simple fix here is to use the unary plus operator to convert the '0' to 0. Then we will fall back to '1' when the field is blank.

[sanchitmalhotra126]

Nice catch! I think this was just copied from the blockly source so I wonder if this is a bug in Blockly as well...