make function more robust

in the Pintos there's a siglum encoded like
```xml
<add xml:id="x38284ce9-f2be-4ca6-a8cb-227d85357ae1" place="rightmar" type="siglum">
    <rend xml:id="x9bbd3666-54ac-4287-b7f7-1d19347c7e14" color="red">Classe III.
        <lb xml:id="x7e7c09da-9ed9-4ed4-9339-75a18868a860"/>Band
        <lb xml:id="xd18991b0-bc11-447b-881a-b732d8f3b3c4"/>V. No. <rend xml:id="x4904459c-a4c4-40a9-beb5-d9f3c21ddeca" rend="underline">80</rend>.</rend>
</add>
```
which features multiple text nodes causing the original function `source:getSiglum#1` to fail.

add/data/xqm/source.xqm

@@ -135,11 +135,8 @@ declare function source:getSiglum($source as xs:string) as xs:string? {
 
     let $doc := doc($source)
     let $elems := $doc//mei:*[@type eq 'siglum']
-    let $siglum :=
-        if(exists($elems)) then
-            ($elems[1]//text())
-        else
-            ()
-
-    return $siglum
+    return
+        if(exists($elems))
+        then $elems[1] => normalize-space()
+        else ()
 };