Use missing instead of is.na to deal with not given parameters

Change-Id: Id7a6153ca3f98cc6e530ca07af8c7a7317251c6a
diff --git a/man/KorAPConnection.Rd b/man/KorAPConnection.Rd
index b62aa80..a3ba70f 100644
--- a/man/KorAPConnection.Rd
+++ b/man/KorAPConnection.Rd
@@ -5,7 +5,7 @@
 \title{Connect to a KorAP server.}
 \usage{
 KorAPConnection(KorAPUrl = defaultKorAPUrl, apiVersion = "v1.0",
-  apiUrl = NA)
+  apiUrl)
 }
 \arguments{
 \item{KorAPUrl}{instead of providing the query and vc string parameters, you can also simply copy a KorAP query URL from your browser and use it here (and in \code{KorAPConnection}) to provide all necessary information for the query.}
diff --git a/man/KorAPCorpusStats.Rd b/man/KorAPCorpusStats.Rd
index 218eb77..71066b2 100644
--- a/man/KorAPCorpusStats.Rd
+++ b/man/KorAPCorpusStats.Rd
@@ -4,7 +4,7 @@
 \alias{KorAPCorpusStats}
 \title{Fetch information about a (virtual) corpus}
 \usage{
-KorAPCorpusStats(con, vc = NA, query = NA)
+KorAPCorpusStats(con, vc, query)
 }
 \arguments{
 \item{con}{object obtained from \code{\link{KorAPConnection}}}
diff --git a/man/KorAPQuery.Rd b/man/KorAPQuery.Rd
index fc68d15..89ea8fe 100644
--- a/man/KorAPQuery.Rd
+++ b/man/KorAPQuery.Rd
@@ -4,9 +4,8 @@
 \alias{KorAPQuery}
 \title{Send a query to a KorAP connection.}
 \usage{
-KorAPQuery(con, query = NA, vc = NA, KorAPUrl = NA,
-  metadataOnly = TRUE, ql = "poliqarp", fields = defaultFields,
-  accessRewriteFatal = TRUE)
+KorAPQuery(con, query, vc = "", KorAPUrl, metadataOnly = TRUE,
+  ql = "poliqarp", fields = defaultFields, accessRewriteFatal = TRUE)
 }
 \arguments{
 \item{con}{object obtained from \code{\link{KorAPConnection}}, that contains all necessary connection information}