SI-7185 Avoid NPE in TreeInfo.isExprSafeToInline

We got there typechecking code with a redundant
layer of Block.

We can't express that in source code, so we test
this with manual tree construction and with XML
literals, which as reported produce such trees.

Author: retronym

test/files/neg/t7185.check

@@ -0,0 +1,7 @@
+t7185.scala:2: error: overloaded method value apply with alternatives:
+  (f: scala.xml.Node => Boolean)scala.xml.NodeSeq <and>
+  (i: Int)scala.xml.Node
+ cannot be applied to ()
+  <e></e>()
+   ^
+one error found

test/files/neg/t7185.scala

@@ -0,0 +1,3 @@
+object Test {
+  <e></e>()
+}