Fix Division By Zero Warning (PHP7 compatibility)

[tremlin]

In our installation magmi didn't work with PHP7 because of a divison by zero error. Calculation mod 0 is no longer supported by PHP7 (only a warning in PHP5). This change prevents this error.

[tremlin]

I haven't tested if this fix is still needed after the latest refactoring of the product import engine.

[tmotyl]

this is not the best way to fix it. The root cause is a bug in the getProp function which doesn't return default value in case of empty string.

around line 936 there is

$pstep = $this->getProp("GLOBAL", "step", 0.5);

And because "step" property is set to empty string and getProp is not handling it correctly $pstep is set to empty string, which then is converted to 0 in the next line

$rstep = ceil(($nitems * $pstep) / 100);

The correct solution for the issue is to fix getProp to return default value (0.5 in this case) when property is an empty string.

[idziakjakub]

This is a fix for this issue:

#516