xparse optional arguments lead to error in tikz

I am trying to create a few simple macros for B-Trees and one of these macros is supposed to create a multipart node tikz based on the number of arguments it gets. Therefore I used the g argument specifier from xparse though tikz always shows the following error:

Package tikz Error: Giving up on this path. Did you forget a semicolon?. ;
Package tikz Error: Giving up on this path. Did you forget a semicolon?. ;
Undefined control sequence. ;

The macro works flawlessly when replacing the gs with ms though then it becomes impossible to differentiate between a multipart node with e.g. 4 segments two of which are empty and an 2 segment node which are all filled.

A kinda minimal working example:

documentclass{article}
usepackage{tikz}
usepackage{xparse}
usetikzlibrary{trees,shapes.multipart}

NewDocumentCommandtest{m g}{
  {#1 IfValueT{#2}{B}}
}
NewDocumentCommandleaf{m m}{
  node[leaf]{#1 nodepart{two} #2}
}
NewDocumentCommandleafOptional{m g}{
  node[leaf]{#1 IfValueT{#2}{nodepart{two} #2}}
}


tikzset{
  btree/.style={
    nodes={rectangle split,rectangle split horizontal=true,draw},
    leaf/.style={
      rectangle split allocate boxes=2,
      rectangle split parts=2,
      rectangle split part fill=white}
  },
}

begin{document}
test{a} % this works
test{a}{something} % as does this
begin{tikzpicture}[btree]
node (root) {test}
  child {leaf{5}{}}
  child {leafOptional{5}} % however this fails
  child {leafOptional{5}{}} % and so does this
  child {node[leaf]{1}}
;
end{tikzpicture}
end{document}

I already found this question TikZ path doesn't work with xparse generated macro which seems to be a similar problem though I am unable to use DeclareDocumentCommand as it does not allow one to use the g/G argument specifier

Addedum

Following @PhelypeOleinik’s comment I changed the code to use NewExpandableDocumentCommand and the argument specifier o o o m however when passing more than one optional argument this leads to tikz not determining the correct node bounding box and tikz draws the edge into the actual node shape.

documentclass{article}
usepackage{tikz}
usepackage{xparse}
usetikzlibrary{trees,shapes.multipart}

NewExpandableDocumentCommandleaf{o o o m}{
  node[leaf]{
    6nodepart{two}8nodepart{three}2nodepart{four}#4
  }
}


tikzset{
  btree/.style={
    nodes={rectangle split,rectangle split horizontal=true,draw},
    level 1/.style={sibling distance=textwidth/4},
    leaf/.style={rectangle split allocate boxes=4}
  },
}

begin{document}
begin{tikzpicture}[btree]
node (root) {test}
  child {leaf{5}}
  child {leaf[a]{5}}
  child {leaf[a][b]{5}}
  child {leaf[a][b][c]{5}}
;
end{tikzpicture}
end{document}

Adding leaf/.style={..., rectangle split part fill=white} seems to fix the symptoms though not the root cause – any possibile solution to use this with transparent nodes?

documentclass{article}
usepackage{tikz}
usepackage{xparse}
usetikzlibrary{trees,shapes.multipart}

NewExpandableDocumentCommandleaf{o o o m}{
    6nodepart{two}8nodepart{three}2nodepart{four}#4
}

tikzset{
  btree/.style={
    nodes={rectangle split,rectangle split horizontal=true,draw},
    level 1/.style={sibling distance=textwidth/4},
    leaf/.style={rectangle split allocate boxes=4}
  }
}

begin{document}
begin{tikzpicture}[btree]
  node (root) {test}
    child { node[leaf]{leaf{5}} }
    child { node[leaf]{leaf[a]{5}} }
    child { node[leaf]{leaf[a][b]{5}} }
    child { node[leaf]{leaf[a][b][c]{5}} }
  ;
end{tikzpicture}

end{document}