Consider generating a non-nullable fields constructor when JSpecify is enabled

[sjohnr]

Consider generating a non-nullable fields constructor when generateJSpecifyAnnotations = true.

Context:

When generateJSpecifyAnnotations = true, the no-arg constructor is never visible. This is intended so applications begin using the public all-args constructor to benefit from nullability annotations or use builders instead. Using javaGenerateAllConstructor = true enables the all-args constructor.

See also gh-900.