Allow for missing Type in signature-field value.
Allow for missing Type in signature-field value.

When determining whether a signature field is signed we were checking for
a value dictionary with Type Sig, but the Type is an optional field. This
commit allows for Type being missing, but checks that at least the value
is a dictionary.

TamirEvan/mupdf,ArtifexSoftware/mupdf,ccxvii/mupdf,TamirEvan/mupdf,TamirEvan/mupdf,TamirEvan/mupdf,sebras/mupdf,ccxvii/mupdf,ArtifexSoftware/mupdf,TamirEvan/mupdf,ArtifexSoftware/mupdf,sebras/mupdf,TamirEvan/mupdf,ccxvii/mupdf,TamirEvan/mupdf,ArtifexSoftware/mupdf,sebras/mupdf,TamirEvan/mupdf,sebras/mupdf,ccxvii/mupdf,sebras/mupdf,ccxvii/mupdf,ccxvii/mupdf,ArtifexSoftware/mupdf,ArtifexSoftware/mupdf,ArtifexSoftware/mupdf,sebras/mupdf,ArtifexSoftware/mupdf