Don't recognize nullable function types as inlinable

#KT-19679 Fixed

compiler/testData/diagnostics/tests/inline/kt19679.kt

@@ -0,0 +1,4 @@
+inline fun test(s: () -> Unit, <!NULLABLE_INLINE_PARAMETER!>p: (() -> Unit)?<!>) {
+    s()
+    p?.invoke()
+}

compiler/testData/diagnostics/tests/inline/kt19679.txt

@@ -0,0 +1,3 @@
+package
+
+public inline fun test(/*0*/ s: () -> kotlin.Unit, /*1*/ p: (() -> kotlin.Unit)?): kotlin.Unit