changed fillNulls and fillNA api to weaken type to be nullable.

Author: Jolanrensen

core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/Nulls.kt

@@ -17,21 +17,27 @@ import kotlin.reflect.KProperty
 
 // region fillNulls
 
-public fun <T, C> DataFrame<T>.fillNulls(cols: ColumnsSelector<T, C>): Update<T, C> = update(cols).where { it == null }
+public fun <T, C> DataFrame<T>.fillNulls(cols: ColumnsSelector<T, C?>): Update<T, C?> =
+    update(cols).where { it == null }
+
 public fun <T> DataFrame<T>.fillNulls(vararg cols: String): Update<T, Any?> = fillNulls { cols.toColumns() }
-public fun <T, C> DataFrame<T>.fillNulls(vararg cols: KProperty<C>): Update<T, C> = fillNulls { cols.toColumns() }
-public fun <T, C> DataFrame<T>.fillNulls(vararg cols: ColumnReference<C>): Update<T, C> = fillNulls { cols.toColumns() }
-public fun <T, C> DataFrame<T>.fillNulls(cols: Iterable<ColumnReference<C>>): Update<T, C> = fillNulls { cols.toColumnSet() }
+public fun <T, C> DataFrame<T>.fillNulls(vararg cols: KProperty<C>): Update<T, C?> = fillNulls { cols.toColumns() }
+public fun <T, C> DataFrame<T>.fillNulls(vararg cols: ColumnReference<C>): Update<T, C?> =
+    fillNulls { cols.toColumns() }
+
+public fun <T, C> DataFrame<T>.fillNulls(cols: Iterable<ColumnReference<C>>): Update<T, C?> =
+    fillNulls { cols.toColumnSet() }
 
 // endregion
 
 internal inline val Any?.isNaN: Boolean get() = (this is Double && isNaN()) || (this is Float && isNaN())
 
-internal inline val Any?.isNA: Boolean get() = when (this) {
-    null -> true
-    is Double -> isNaN()
-    is Float -> isNaN()
-    is AnyRow -> allNA()
+internal inline val Any?.isNA: Boolean
+    get() = when (this) {
+        null -> true
+        is Double -> isNaN()
+        is Float -> isNaN()
+        is AnyRow -> allNA()
     is AnyFrame -> isEmpty()
     else -> false
 }
@@ -50,17 +56,18 @@ public fun <T, C> DataFrame<T>.fillNaNs(cols: ColumnsSelector<T, C>): Update<T,
 public fun <T> DataFrame<T>.fillNaNs(vararg cols: String): Update<T, Any?> = fillNaNs { cols.toColumns() }
 public fun <T, C> DataFrame<T>.fillNaNs(vararg cols: KProperty<C>): Update<T, C> = fillNaNs { cols.toColumns() }
 public fun <T, C> DataFrame<T>.fillNaNs(vararg cols: ColumnReference<C>): Update<T, C> = fillNaNs { cols.toColumns() }
-public fun <T, C> DataFrame<T>.fillNaNs(cols: Iterable<ColumnReference<C>>): Update<T, C> = fillNaNs { cols.toColumnSet() }
+public fun <T, C> DataFrame<T>.fillNaNs(cols: Iterable<ColumnReference<C>>): Update<T, C> =
+    fillNaNs { cols.toColumnSet() }
 
 // endregion
 
 // region fillNA
 
-public fun <T, C> DataFrame<T>.fillNA(cols: ColumnsSelector<T, C>): Update<T, C> = update(cols).where { it.isNA }
+public fun <T, C> DataFrame<T>.fillNA(cols: ColumnsSelector<T, C?>): Update<T, C?> = update(cols).where { it.isNA }
 public fun <T> DataFrame<T>.fillNA(vararg cols: String): Update<T, Any?> = fillNA { cols.toColumns() }
-public fun <T, C> DataFrame<T>.fillNA(vararg cols: KProperty<C>): Update<T, C> = fillNA { cols.toColumns() }
-public fun <T, C> DataFrame<T>.fillNA(vararg cols: ColumnReference<C>): Update<T, C> = fillNA { cols.toColumns() }
-public fun <T, C> DataFrame<T>.fillNA(cols: Iterable<ColumnReference<C>>): Update<T, C> = fillNA { cols.toColumnSet() }
+public fun <T, C> DataFrame<T>.fillNA(vararg cols: KProperty<C>): Update<T, C?> = fillNA { cols.toColumns() }
+public fun <T, C> DataFrame<T>.fillNA(vararg cols: ColumnReference<C>): Update<T, C?> = fillNA { cols.toColumns() }
+public fun <T, C> DataFrame<T>.fillNA(cols: Iterable<ColumnReference<C>>): Update<T, C?> = fillNA { cols.toColumnSet() }
 
 // endregion