Use prefix-less prepend methods directly to reduce branches in String concat expressions

Binding null values to String concat prependers means the JIT sometimes generate suboptimal code. By disallowing null (and empty) values in the argument + prefix prepender and use argument-only prependers directly for null and empty we reduce branches, simplify the JITs work and get more consistent performance on some microbenchmarks, for example BigDecimals.testSmallToEng added by JDK-8315585