Prior to this commit, HtmlUnitRequestBuilder would incorrectly attempt
to decode null values for query parameters (i.e., query parameters such
as 'error' in 'http://example.com/login?error') which resulted in a
NullPointerException since URLDecoder.decode() does not support null
values.
This commit fixes this issue by ensuring that HtmlUnitRequestBuilder
only attempts to decode non-null query parameter values.
Issue: SPR-13524
283fc94572