fix(boa): fix stringpad abstract operation

[neeldug]

This Pull Request fixes/closes #1306.

It changes the following:

• renames filter to filler in accordance with spec
• adds empty string check to StringPad function

[Razican]

Test262 conformance changes:

Test result	master count	PR count	difference
Total	78,897	78,897	0
Passed	26,940	26,945	+5
Ignored	15,628	15,624	-4
Failed	36,329	36,328	-1
Panics	1	0	-1
Conformance	34.15%	34.15%	+0.01%

Fixed tests:

test/built-ins/Map/prototype/keys/returns-iterator.js (previously Panic)
test/built-ins/String/prototype/padStart/fill-string-empty.js [strict mode] (previously Ignored)
test/built-ins/String/prototype/padStart/fill-string-empty.js (previously Ignored)
test/built-ins/String/prototype/padEnd/fill-string-empty.js [strict mode] (previously Ignored)
test/built-ins/String/prototype/padEnd/fill-string-empty.js (previously Ignored)

[Razican]

Looks good to me. But note that it only fixes two of the bugs mentioned in the issue. We still have some more to fix, so this won't close the issue.

Edit: nevermind, I just saw the rest of the fixes. If all of them are fixed, we can close the issue :) we just need to take care when merging, and make sure that all have been fixed.