RFR: 8277412: Use String.isBlank to simplify code in sun.net.www.protocol.mailto.Handler

[Andrey Turbanov]

All this manually written code actually can be replaced with single String.isBlank() call.
`file` variable is guaranteed to be non-null.

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Commit messages:
 - [PATCH] Use String.isBlank to simplify code

Changes: https://git.openjdk.java.net/jdk/pull/6372/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6372&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277412
  Stats: 26 lines in 1 file changed: 0 ins; 21 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6372.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6372/head:pull/6372

PR: https://git.openjdk.java.net/jdk/pull/6372