Pass empty nodes to Dependency Graphs

[AngledLuffa]

Allow for passing "empty nodes" to Dependency Graphs (SemanticGraph) This will allow for running Semgrex queries over those documents.

An example where this will be relevant is in the Estonian sentence

# sent_id = ewtb2_000035_15
# text = Ja paari aasta pärast rôômalt maasikatele ...
1       Ja      ja      CCONJ   J       _       3       cc      5.1:cc  _
2       paari   paar    NUM     N       Case=Gen|Number=Sing|NumForm=Word|NumType=Card  3       nummod  3:nummod        _
3       aasta   aasta   NOUN    S       Case=Gen|Number=Sing    0       root    5.1:obl _
4       pärast  pärast  ADP     K       AdpType=Post    3       case    3:case  _
5       rôômalt rõõmsalt        ADV     D       Typo=Yes        3       advmod  5.1:advmod      Orphan=Yes|CorrectForm=rõõmsalt
5.1     panna   panema  VERB    V       VerbForm=Inf    _       _       0:root  Empty=5.1
6       maasikatele     maasikas        NOUN    S       Case=All|Number=Plur    3       obl     5.1:obl Orphan=Yes
7       ...     ...     PUNCT   Z       _       3       punct   5.1:punct       _

Also need to add a mechanism for passing multiple graphs using the same nodes and searching over multiple graphs at once. This will allow for Semgrex queries over those enhanced graphs. Similarly, will eventually need to upgrade Ssurgeon to process multiple graphs at once