BUG: Do not create empty build-tree RPATH

The fix for issue #9130 appends ':' to the end of the build-tree RPATH
unconditionally.  This changes the fix to add ':' only when the RPATH is
not empty so that we do not create a build-tree RPATH with just ':'.  An
empty RPATH produces no string at all, so there is no chance of merging
with a symbol name anyway.

Source/cmComputeLinkInformation.cxx

@@ -1708,10 +1708,13 @@ std::string cmComputeLinkInformation::GetRPathString(bool for_install)
   // If the rpath will be replaced at install time, prepare space.
   if(!for_install && this->RuntimeUseChrpath)
     {
-    // Always add one trailing separator so the linker does not re-use
-    // the rpath .dynstr entry for a symbol name that happens to match
-    // the end of the rpath string.
-    rpath += this->GetRuntimeSep();
+    if(!rpath.empty())
+      {
+      // Add one trailing separator so the linker does not re-use the
+      // rpath .dynstr entry for a symbol name that happens to match
+      // the end of the rpath string.
+      rpath += this->GetRuntimeSep();
+      }
 
     // Make sure it is long enough to hold the replacement value.
     std::string::size_type minLength = this->GetChrpathString().length();