a missing fractionExpr should not always automatically equal 1

mixed expression bcs are defined with up to three expressions:

• valueExpr
• gradientExpr
• fractionExpr

Missing values:

• missing valueExpr treated as zero
• missing gradientExpr treated as zero
• missing fractionExpr treated as one

Treating a missing fractionExpr like "1" makes sense for handling value expressions (when gradientExpr is missing), but does not really make sense when valueExpr itself is missing. For example,

{
    gradientExpr  " .... ";
}

The user would rightly expect that the gradient expression should be used, not ignored.